systematic literature review homelessness

• Open access
• Published: 25 April 2022

A qualitative systematic review on the experiences of homelessness among older adults

• Phuntsho Om 1 , 2 ,
• Lisa Whitehead 1 , 3 ,
• Caroline Vafeas 1 &
• Amanda Towell-Barnard 1  

BMC Geriatrics volume  22 , Article number:  363 ( 2022 ) Cite this article

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Adults who experience homelessness for an extended period of time also experience accelerated ageing and other negative impacts on their general health and wellbeing. Homelessness amongst older adults is on the rise, yet there are few systematic reviews investigating their experiences. Thus, this review classifies and synthesises qualitative research findings of studies published between 1990 to 2020 that have examined the needs and challenges of homeless older adults to elucidate their journey of homelessness. Seven papers met the requirements for inclusion. Three main themes were identified in the review: - (1) Pathways to homelessness, (2) Impact of homelessness, and (3) Outcomes and resolutions. This review collates current evidence on what is known about the experience of homelessness among older adults. In this study, homeless older adults identified a wide range of challenges associated with the experience of homelessness.

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The population globally is ageing. Although, ageing is truly a triumph of development, this demographic change presents both advantages and challenges. The concept of successful ageing is to “add life to years” rather than adding days to life and is about maximizing wellbeing and happiness for the older adult [ 1 ]. The risk of developing physical and mental health issues among older adults along with associated costs are linked to a higher demand for health and social care [ 2 ].

Theories on ageing have been developed with the goal of understanding the ageing process and how best to support “healthy ageing at home” and “ageing-in-place” [ 3 ] however these do not consider older adults who do not live in a supportive environment or adults who are homeless. The home setting can be a place associated with poor subjective well-being and some older adults may feel compelled to leave the home setting as a result [ 4 ].

There is no consistent definition of homelessness, rather it has been confined to socio-historical, geographical, and cultural contexts from which the term is drawn [ 5 ]. Homelessness can be defined by a range of categories: absolute and or hidden with homelessness defined as sleeping in parked cars or parks, in emergency shelters, or in temporary shelters (couch surfing) with no or minimal health and safety requirement standards, and risk to personal safety [ 6 , 7 , 8 ]. This includes people residing in sub-standard housing such, as single-room occupancy hotels, or cheap boarding houses, as well as low-cost tiny, lodgings with minimal amenities [ 9 , 10 ].

There is an increasing rise in homelessness among older adults and older homeless adults have been identified as the “new homeless”, a “forgotten group” and a “hidden group” [ 9 , 11 , 12 ].

The reasons for homelessness amongst older adults are diverse. These can include: the impact of natural disasters; the availability of affordable housing, including rising rental costs, a decline in social welfare and support programs; financial insecurity; a lack of social amenities; and increasing rates of mental health issues, combined with various addictions, including gambling [ 2 , 9 , 11 , 12 , 13 , 14 , 15 , 16 ]. In addition to this, family relationship breakdowns, or the death of loved ones, can cut people’s social connections, resulting in older adults experiencing homelessness for the first time. This displacement of older more vulnerable adults can lead to deprivation including the basic need for a place they can relate to as home, subsequently leaving them homeless [ 3 , 7 , 15 , 17 ].

Molinari, Brown and Frahm et al. (2013) found homelessness was unanimously perceived as a humiliating experience by homeless older adults [ 13 ]. According to a survey conducted by the United States Department of Housing and Urban Development, over 15% of 634,000 homeless individuals were 50 years or older, where the number of homeless people aged over 65 has been projected to double by 2050 [ 13 ]. The same survey reported that in the United States alone, adults as young as 50 years of age were facing challenges of homelessness, effectively accelerating ageing processes. Further to this, homeless older adults face a greater threat of age-related disease burden, where they are more likely to experience: functional, auditory, visual, and neurological impairments, frailty, emotional distress, and urinary incontinence, at higher rates than in the general community [ 18 ].

Similarly, van Dongen et al. have reported within a longitudinal cohort study, that older homeless adults, unlike their younger counterparts, reported a higher incidence of cardiovascular disease and visual problems, as well as reporting limited social support from family and friends or acquaintances, and limited medical or hospital care use in the past [ 19 ].

However, there is limited published research identifying the distinct needs of homeless older adults. This is a critical gap in the literature, where a deeper understanding of the experiences of older adults who have been or are currently homeless is required.

The main aim of this qualitative systematic review is to synthesise the evidence on the experience of homelessness of older adults.

Using Joanna Briggs Institute (JBI) guidelines, a meta-synthesis of global qualitative evidence was undertaken. Studies with titles and abstracts that met the analysis goals were retrieved and chosen, based on inclusion and exclusion criteria. These studies were further appraised to evaluate methodological validity by analysing evidence relevant to viability, appropriateness, meaningfulness, and effectiveness [ 20 ]. Qualitative and mixed-method studies with ample qualitative data in their results sections to allow secondary data analysis met the inclusion criteria. The sample comprised of older adults aged between 45 and 80 years that had experienced homelessness for at least one period. The search was restricted to studies that were published in English and available in full-text form, where studies with participants below 45 years, older adults in housing facilities, and aged care residents were excluded.

Search methods

This analysis followed the Joanna Briggs Institute (JBI) method for systematic reviews [ 20 ]. A qualitative assessment and review instrument (JBI-QARI 10 item tool) [ 20 ] was used to facilitate the meta-synthesis. Results from the studies were extracted, categorised, and synthesised. Searches were conducted in PsycINFO, Web of Science, Google Scholar, Medline, PubMed, and CINAHL using appropriate search terms. Additionally, important citations were searched from reference lists of relevant articles. Searches were limited to published studies from 1990 to 2020 (see Fig.  1 ).

PRISMA flowchart

Quality appraisal

Two reviewers independently assessed 21 articles for methodological quality in their design, conduct and analysis using the JBI-QARI 10 item tool [ 20 ]. Any discrepancies were discussed within the team. Out of the 21 articles, seven were included in the synthesis. Each selected study was re-read several times, discussed within the review team and data were abstracted for interpretation.

Data abstraction

Findings relating to both current and past experiences of homelessness among older adults were extracted from the seven selected studies. A total of 56 findings were extracted. Each finding was reviewed and further compared and manually coded to identify themes. Table  1 lists the author and year, sample size, design, setting, and participant characteristics of the selected studies.

Analysis of the seven reviewed articles was carried out using the qualitative evidence synthesis method [ 20 ] developed by JBI (2014). Qualitative findings from each study were first read and reread, followed by an identification of common themes. Recurring themes across studies were then grouped together in a meta-synthesis of the findings. This process comprised critical appraisal, data extraction, analysis, and a meta synthesis involving organisation and categorisation through decoding and encoding of the extracted data to produce a final summation of the findings. The qualitative evidence summation and synthesis were deliberated, cross-checked, and then reviewed by all the authors.

Of the seven studies identified for review (see Table 1 above), four studies directly explored pathways to homelessness amongst older adults. Individual study sample sizes ranged from 14 in Reynolds, et al. (2016) [ 21 ] to 60 in Viwatpanich (2015) [ 24 ]. Three studies applied in-depth face to face interviews, with three studies using semi-structured interviews, and one study conducting focus groups to collect data. The studies were conducted in three countries: Canada, USA, and Thailand.

Data synthesis commenced using open descriptive coding to search and identify concepts and finding relationship between them. Next using an interpretive process, the meaning units were categorised within each domain using labels close to the original language of the participants. The categorization of the data for each case was then followed by a cross-case analysis that examined the similarities and differences. Following categorisation, themes were conceptualised for each category. An overarching theme was identified: ‘the journey of homelessness’. Within this context, three core themes were identified: 1) Pathways to homelessness; 2) impact of homelessness; and 3) outcomes and resolutions, where each of these 3 themes had relevant sub-themes. (see Fig.  2 ).

The Journey of Homelessness Model

The conceptual model depicted in Fig. 2 represents the overarching theme of the ‘journey to homelessness,” and key concepts and relationships between variables from the synthesis of the literature. Unlike other conceptual models that involve causal and directional relationships, this model is both directional and non-hierarchical. The model illustrates the pathways to homelessness, the associated impacts of homelessness and the outcomes of homelessness. The following section explores the three themes and sub-themes in more detail.

Theme 1. Pathways to homelessness

The causes of homelessness were shown to be multifaceted, where pathways to homelessness revolved around a combination of individual, social, and structural factors. The reviewed data suggested that becoming homeless involved two distinct pathways: one that was gradual and one that was rapid.

Sub-theme 1.1: gradual pathway to homelessness

Findings from six studies contributed to this subtheme. This sub-theme captured the factors contributing to gradual pathways into homelessness amongst older people. These factors were identified as accelerated ageing, poverty, rising housing costs, failing and uncommitted social security systems, a lack of social programs and services, social distress, rural-urban migration, substance abuse and addiction, as well as estrangement from family or lack of living relatives [ 13 , 14 , 21 , 22 , 23 , 24 , 25 , 26 ].

The following quotations from these studies illustrate both estrangement from family and the impact of a lack of support from social services:

Many conflicts we had at that time, we never talked … never talked in normal way … nothing clear between us, emotion never came clear...they did not want to talk to me, not even to look at my face … I could not stand it, I surrendered. Beating and scolding by descendants is not in our tradition, no respect, if they did not want me to stay with them, I moved out [ 24 ] .

I submitted applications for low-income housing, I’ve been on the waiting list, seven years is a long time, especially at my age [ 22 ] .

Personal vulnerability to difficult familial relationships, neglected needs and unstable housing were the most cited causes of homelessness amongst these older adults [ 3 , 9 , 14 , 25 ].

Two studies [ 15 , 21 ] described a pathway to homelessness as related to alcoholism and drug abuse, as highlighted in the following quotation:

I got into crack cocaine, I got into hooking, I got into anything you could think of I guess . . . So it was my addictions that brought me down, and unhealthy relationships [ 25 ] .

Feeling ‘homeless at home’ [ 27 ] due to loneliness was noted by some older adults as their reason for ‘living on the streets’. For example, homeless older adults that experienced social rejection and conflicts with housing management, neighbours, and roommates, noted this to ultimately lead them to homelessness. For example, one participant stated, “I have lived alone and never really felt at home, because to me home is a place that includes other people, your family” [ 23 ].

Sub-theme 1.2: rapid pathway to homelessness

Some older adults described the process of homelessness as ‘rapid’. A rapid pathway to homelessness was associated with abrupt life changes such as losing a loved one, divorce, and the impact of these losses on their lives. The two quotes below highlight rapid pathway process:

Losing them, let’s just say it evaporates over time. It’s the fact that I wake up like I am here that I can’t accept … homeless … in the street. I sold everything, every single thing! I never thought I’d end up like this. It’s like starting from zero [ 23 ].

I had a wife, then she died, I did not know where to go, what to do, I turned homeless [ 24 ] .

Older adults that faced a series of losses and a rapid deprivation of social support systems noted the experience of disrupted circumstances. Accordingly, they noted their fear of losing their independence and ‘sense of self’ resulted in their resistance to any help that was offered, in turn contributing to their homelessness.

Theme 2. Impact of homelessness

Findings from five studies contributed to this subtheme. Homelessness and ageing were presented to form a ‘double jeopardy’ where homelessness aggravated the challenges of old age [ 15 , 21 , 22 , 23 , 24 ].

This theme included the subthemes of: unmet needs, coping strategies, and the realities of housing availability.

Sub-theme 2.1: unmet needs

‘Unmet needs’ amongst older homeless adults were categorised as involving physical, emotional and social needs leading to despair and destitution. As this quote below highlights:

I’m supposed to get a pneumoscopy, but where am I, where do I stay? How can they get a hold of me? I don’t have money to get around [ 15 ] .

Sub-theme 2.1.1: lack of physical wellbeing

Findings from six studies [ 14 , 15 , 18 , 21 , 22 , 24 ] contributed to this subtheme. Physical decline and physical disability were described as exacerbated by the experience of being homeless. Participants described a relationship between age and frailty, fatigue, poor physical health, and impaired mobility while homeless, as these quotes demonstrate:

Ah! Walking all day, for me, it’s very hard on the body, ok. Sleeping outside on a park bench, that’s very, very hard on the body. The bones, the humidity. Just leaving in the morning and then not going to work. … You’re always faced with the outdoors, and always faced with walking, walking. It’s not easy walking from downtown [ 15 ] .

My health was very poor. I was very prone to pneumonia. I was taken out of the shelter in the ambulance and it was later determined that I had actually contracted tuberculosis [ 22 ] .

At that time, I got Psoriasis, I knew that it was disgusting … . It looked scary. I am much too old. It is so difficult to find a job … nobody needed me … so I decided to stay and sleep here [ 24 ] .

Homelessness in later life was shown to often be linked to a multitude of health problems. Most studies described older homeless people as living with physical health problems including chronic diseases such as hypertension, diabetes, bone and joint diseases, respiratory illness, and skin diseases [ 14 , 21 , 22 ]..

Sub-theme 2.1.2: lack of emotional wellbeing

Findings from five studies contributed to this subtheme. Accordingly, homelessness was described as contributing to poor emotional health related to social exclusion and isolation amongst older adults. Further, homelessness was associated with cognitive impairment, stigma, shame, stress and anxiety, as well as depression amongst homeless older adults [ 15 , 21 , 24 , 25 ]. Homelessness was described as a humiliating and degrading experience, as evident in these quotes:

At my age, I don’t see life ahead of me anymore. You see, I don’t know, I don’t see the end of the tunnel, … … It’s as if I wanted to erase myself [ 15 ] .

All I could think about was suicide. How did I end up here? When I think a lot to myself, what the hell am I doing? [ 23 ] .

Feelings such as shame, demoralisation, and loss of dignity were described and these impacted on emotional health.

Opportunities to improve emotional wellbeing were rarely described, however one example stood out as an exception and this was related to volunteering:

One thing I didn’t expect was when I helped people with whatever issues they were having on their bicycle, I really enjoyed that. It gave me a chance to teach someone [ 25 ] .

Examples such as these were rare, with social exclusion and the lack of opportunity to contribute and connect with others more commonly described.

Sub-theme 2.1.3: lack of social relationships

Findings from four studies contributed to this subtheme. Social relationships were described as central to creating a life that had meaning and familial interactions. Disconnection from loved ones was associated with feelings of unhappiness [ 13 , 15 , 27 ], while companionship was shown to improve wellbeing [ 25 ]. Social relationships were shown to decline, leading to the experience of social exclusion and isolation.

I am a walking dying woman. I walk until I can’t walk anymore, and then I sit. The busses pass me by. We are untouchables and I do not think anybody’s going to do anything about it [ 25 ] .

At my age, I don’t see life ahead of me anymore. Because everywhere I go: “Ah! He’s homeless.” It is as if I wanted to erase myself. I think that it’s more “society,” as such, that rejects homeless people [ 15 ] .

I think that living homeless, you exclude yourself, and a lot of other people exclude you. I was on the other side before becoming homeless. So, you know, the perception that people have, it plays a big part. … So that together makes it so that, if you don’t have family either, let’s say, you don’t have … close friends or a strong social network. Well, you experience all that, you live with loneliness and isolation [ 15 ] .

Sub-theme 2.2: impaired coping strategies

Findings from four studies contributed to this subtheme. Older homeless adults described a range of factors as impacting their ability to cope. These included moving to shelters, challenges to adapt to their unique requirements, limited housing options, limited income supports, social exclusion, isolation, and a lack of coordination and access to community health and support services [ 13 , 15 , 23 , 25 ].

As the quote below shows, there were expressions about the fear of homelessness and how long it will last:

Struggling to get your basic needs met, scrounging, just trying to get by as best I can, and feeling desperation, humiliation, despair, a shocking feeling, full of fear, and turmoil. What’s tomorrow gonna bring? Why am I in this situation? How do I get out of it? [ 13 ]

Coping with the harsh realities of homelessness in later life was described as being increasingly challenging for most older adults because older homeless individuals experience mental health disorders and acute or chronic physical illnesses.

Sub-theme 2.3: realities of housing availability

Findings from three studies [ 13 , 15 , 23 ] described the challenges experienced in accessing housing services and fulfilling requirements for safe, secure, and affordable housing. This theme captured impacts of poor coordination and communication between homeless veterans and housing intervention providers in regard to information for service availability, gaining access to homeless shelters and a lack of training and education by some housing providers especially with regard to homelessness.

He … got this rule book and threw it at me. Find a place! [ 13 ]

You know, I’m 60, I’m not 20 anymore. So that’s what makes you tired, you get stressed. So, after that, they give you pills as a solution. I told the doctor, sorry I didn’t come here for pills, I came for housing [ 23 ] .

I submitted applications for low-income housing, I’ve been on the waiting list, seven years is a long time, especially at my age [ 23 ] .

I want a space where I can be well. I wasn’t well when I was young. I’ve never been well anywhere. I need a simple place … where I can have peace, and quiet … but not be all alone [ 15 ] .

Older homeless adults described a need to create stability and escape homelessness through the provision of services, and in particular, housing. Older adults described how oscillating in and out of shelters prevented senses of safety, stability, or autonomy.

Theme 3. Outcomes and resolutions

In four studies [ 13 , 15 , 21 , 24 ] homeless older adults described how the outcomes and resolutions of homelessness involved overcoming both complex challenges and habituations. This theme encompassed the finding of directions and strengths to improve difficult situations and overcome challenges that occurred at the intersection of homelessness and ageing.

Three subthemes were identified within this theme: building resilience, strength, and hope; seeking spiritual meaning; and exiting the cycle of homelessness.

Sub-theme 3.1: exiting the cycle of homelessness

Some older adults moved out of the phase of homelessness and described facilitators and barriers to this transition whilst other described choosing to stay homeless until the end of their lives.

Sub-theme 3.1.1: factors facilitating the exit

Two studies [ 13 , 15 ] contributed to this sub-theme, where older adults described means of overcoming challenges and establishing priorities in order to exit homelessness in later life. The results suggested that the creation of autonomy, flexibility, and privacy helped people feel belonging and often this meant living in a place where they could continue to drink and/or occasionally use drugs, have access to a health system to manage health problems; and have access to food and shelter facilitated exits.

They listen to you and they help you with . . . your transition, your program. You sit down and you work the program out with them;” “If you have a question, you can walk in anytime and ask them what’s going on [ 13 ] .

In the next couple years, I hope to find myself an apartment for the few good years I have left, before the big pains of “aging” come [ 15 ] .

Fulfilling financial support, housing and health care services was identified facilitate older adults exiting homelessness.

Sub-theme 3.2: remaining homeless

Some older adults experienced homelessness at a younger age and described continuing to be homeless in older age, where they oscillated between living in shelters and on the streets.

I am used to being in this way, moved from place to place … me alone, without father and mother since childhood … it become normal and I feel happier, than to stay with others [ 24 ] .

It’s just a continual cycle. I just got sucked down into it, you know. It’s hard to describe because when I found myself there, I was just like, wow. How did I get here? [ 21 ]

Participants described the chronic nature of homelessness as involving a challenge of disentangling themselves from the cycle of homelessness. A lack of tailored intervention programs to respond to homelessness in later life also prevented older adults from exiting homelessness.

Sub-theme 3.2.1: perceived barriers to exiting homelessness

In two studies [ 21 , 24 ], older adults described experiences of vulnerabilities and challenges to exiting homelessness. Shelters were described as constraining and not being able to adapt to the unique needs of older adults. Where limited housing options were seen as available, income supports were described as limited, with a lack of coordinated and, accessible community health and social support services, impacting on participants’ ability to ‘feel in place’.

My health pretty much stayed the same as when I was homeless. The conditions I have aren't gonna improve [ 22 ] .

It’s harder to keep a place, especially when you keep falling back in the same circle and you’re in the same crowd. I am finding out today, you keep falling back in the same circle, the same circle is not gonna change [ 21 ] .

One participant described the difficulty of obtaining employment as a barrier to exiting homelessness:

You know being 50 years old, it’s going to be really difficult to be able to reintegrate into the workforce [ 21 ] .

Housing facilities and transition to housing shelters were shown to present challenges for homeless older adults. A lack of privacy, autonomy, rigid rules, and challenging interpersonal relationships within housing and shelter programs were identified as leading older adults to feel homeless at home.

Sub-theme 3.3: building resilience and strength

This sub-theme captured the life lessons, resilience, strength, and hope of older homeless adults, described as having formed through experiences and skills developed whilst living on the streets. This theme also suggests how individuals cope with difficult symptoms related to social support and, addiction, relying on positive things learned while living with other homeless people on the streets. Some older adults chose to stay homeless accepting homelessness as their fate.

In the next couple years, I hope to find myself an apartment for the few good years I have left, before the big pains of “aging” come. I really want a normal life, get up in the morning, go to work, think about vacation. Hang out with other people … I don’t have a girlfriend but would like to start a life with someone else [ 15 ] .

What does ageing mean to you, getting older on the streets? A: Experience. Q: Ok. A: Wisdom. Q: Getting older on the streets, that’s how you see it, it’s the wisdom that you have gained. A: Yeah, that’s where I learned to be wise. Because there are several people who told me I am wise [ 15 ] .

I think because of karma … I accept it as punishment from bad deeds in my former life, but only in this life okay! Next life I am looking forward for a normal life, like others [ 24 ] .

Most studies [ 3 , 8 , 13 , 17 ] cited that wisdom, experience, and optimism were necessary in order to help older adults exit homelessness. Optimism instilled future hope and self-worth back into the self-esteem of homeless older adults.

Sub-theme 3.3.1: seeking spiritual meaning

In two studies [ 24 , 25 ], older adults described finding meaning in life through adopting and accepting religious faith with a belief to achieve higher self-actualisation.

I want to be closer to Dhamma (Buddhist teaching), I want to be a monk till I die [ 24 ] .

Meditate, just being by myself. Living the night, just being alone and listening to my music, that makes [my pain] feel better. I like jazz but I just listen to my music, just go away to myself. That makes me feel - I like being alone. I love being alone [ 25 ] .

When I feel [anger over my situation] I go to the water and I pray hard. I just start praising God until I can feel the spirit come over me to comfort me. I pray until He comes and allows his spirit to wrap his arm around me; I feel a lot better. A psychiatrist can’t tell me what’s wrong with me. For someone to try to help would mean a lot. I do not have nobody but to trust God. He’s my only psychiatrist [ 25 ] .

Homeless older adults recognised and confirmed that psychosocial and existential symptoms caused as much distress as physical symptoms triggering negative changes in personality, energy, and motivation. Some homeless older adults viewed their age as a source of strength, wisdom, and experience in learning to manage their symptoms, describing themselves as survivors who had overcome significant hardships. Higher levels of wellbeing were likely to be achieved when older people sought spiritual meaning through religion, socialising, reading, meditating, volunteering, and introspection practices.

This review synthesised evidence generated from qualitative studies to provide a glimpse into the experiences of homeless older adults. The review has shown that while drivers related to entry into homelessness were diverse, two distinct trajectories underpinned the experience of becoming homeless amongst older adults. Older people that faced a sudden series of losses that completely overturned their circumstances fell into the ‘rapid pathway’ to homelessness. Participants on a ‘gradual pathway’ were shown to become homeless due to a range of factors, for example - addiction problems, physical and mental health issues, relationship break-ups, foster care, poverty, unemployment, and greater housing instability [ 13 , 24 ]. Further to this, homeless older adults were shown to include a significant percentage of separated, divorced, or single individuals [ 28 ]. Likewise becoming single in later life was shown to be associated with homelessness amongst older people. Other studies found that ageing, its associated factors and a lack of stable housing were prominent reasons for homelessness [ 15 , 22 , 23 ].

Housing was perceived to offer a sense of security and a stable environment conducive for safe ageing. Further, housing was identified as offering protection from harsh weather and other dangers. Similar accounts relaying how the health of homeless older adults declined during episodes of homelessness was also reported [ 9 ]. Stable housing played an influencing role in physical health and general wellbeing. Although homeless older adults expressed satisfaction with life, they linked secure housing with healthy dietary habits, proper sleep patterns, enhanced self-care and reduced feelings of stress and anxiety [ 22 ]. In addition, this review found that most homeless older adults were more able to prioritise their health care needs when other necessities such as food and shelter were met. However, research has also suggested that living in scattered-site apartments can reinforce the process of social exclusion, and thus they are not appropriate for older adults living alone, with regard to their additional health and social needs [ 3 , 10 , 28 ].

Ageing intensified the adversities of homelessness experiences and presented a twofold risk where homelessness aggravated the challenges of old age and vice versa [ 15 ]. Old age and its associated conditions intensified older adults’ perceptions of homelessness later in life, including feelings of shame, anxiety, and worry. Studies by Cohen [ 9 ], Kwan, Lau and Cheung [ 29 ], and Molinari et al. [ 13 ], have unanimously shown older adults to perceive homelessness as a dehumanising experience. Homelessness was described as: struggling “to get your basic needs met,” “scrounging, just trying to get by as best I can,” and feeling “desperation,” “humiliation,” “despair,” “a shocking feeling,” “full of dread, turmoil,” “what will tomorrow bring? why am I in this predicament and how can I get over it?” [ 13 ]. For most participants, homelessness was not a preferred option.

The limitations of this review include the predominance of data collected in North America which may reduce the generalisability of the findings. Another drawback is that it presents only a cursory review of issues related to gender, race, and ethnicity. Finally, the qualitative data analysis applied by the majority of studies here is subjective, where outcomes could be affected by authors’ personal biases.

Despite these limitations, the review has conceptualised two divergent pathways into homelessness in later life, as well as the impacts of homelessness, drawing attention to a greater understanding of homelessness experienced by older adults.

The review sought to provide insight into the needs of homeless older adults. Awareness of the complexities faced by homeless older adults need to be acknowledged if policy and research are to support the population and improve access to resources and support. The review has highlighted areas for future research to expand knowledge and understanding of the unique needs and challenges of homeless older adults.

Synthesis of seven studies resulted in the identification of an overarching theme relating to the ‘journey of homelessness’ and three major themes, each with subthemes, to describe older adults’ experiences of homelessness. A broad range of diverse settings, cultures, and countries with a particular focus on homelessness in later life were included. The review has revealed homogeneity of experiences amongst homeless older adults, with the need for access to appropriate and affordable housing and adequate support systems.

The findings have identified pathways to homelessness require different prevention and support measures. People in the study who described a gradual pathway needed social support to address distress, which might have helped them avoid losing their homes. Those individuals with rapid pathways unanimously concluded that homelessness could have been avoided if independence and self-sufficiency were less regarded as a norm by society.

Availability of data and materials

The authors declare that all data generated or analysed during this study are included in this published article.

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Acknowledgements

We would like to acknowledge Lisa Webb, Librarian, Edith Cowan University Library for her support in the literature search and Dr. Michael Stein, HDR Communication Advisor, Edith Cowan University for editing.

There are no separate funding source for this review as it is part of my full-time PhD study with the School of Nursing Midwifery, Edith Cowan University.

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PO, LW, CV, and ATB substantially contributed to the conception and design of the article. All authors critically appraised the searched literature, discussed each item in the appraisal instrument for each study included in the review and interpreting the relevant findings. The primary author PO drafted the article and LW, CV and ATB revised it critically for important intellectual content. The author(s) read and approved the final manuscript.

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Om, P., Whitehead, L., Vafeas, C. et al. A qualitative systematic review on the experiences of homelessness among older adults. BMC Geriatr 22 , 363 (2022). https://doi.org/10.1186/s12877-022-02978-9

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• Homelessness
• Older adults
• Health and wellbeing
• Qualitative
• Systematic review

BMC Geriatrics

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Homelessness, health and the policy process: A literature review

Affiliations.

• 1 University of Sydney Menzies Centre for Health Policy, Faculty of Medicine & Health, Sydney 2006, Australia. Electronic address: [email protected].
• 2 University of Sydney Menzies Centre for Health Policy, Faculty of Medicine & Health, Sydney 2006, Australia.
• PMID: 31522758
• DOI: 10.1016/j.healthpol.2019.08.011

Homelessness has serious consequences for the health of people experiencing homelessness, and presents a challenge to the provision of quality care by health services. Policymaking to address homelessness, as with other social determinants of health (SDH), is complicated by issues of complex causation, intersectoral working and the dominance of biomedicine within health policy. This paper investigates how policies addressing homelessness have been explored using formal policy process theories (PPT). It also examines how health (as an actor and an idea) has intersected with the issue of homelessness reaching policy agendas and in policy implementation. A systematised search of academic databases for peer-reviewed literature from 1986 to 2018 identified six studies of homelessness policy change from Australia, Canada, France and the United States. PPT were able to articulate the interplay of actors, ideas and structures in homelessness policymaking. When the health sector was involved, it tended to be in terms of healthcare service utilisation rather than a broader public health framework emphasising structural social determinants of homelessness. Tensions between differing the priorities of local homelessness actors and a biomedical evidence-based policy paradigm were noted. Future policy action on homelessness requires new models of intersectoral governance that account for the complexity of health determinants, a health workforce enabled to engage with the SDH, and meaningful inclusion of those with lived and living experience of homelessness in policy formulation.

Keywords: Homelessness; Intersectoral working; Policy process; Political science; Social determinants of health.

Copyright © 2019 Elsevier B.V. All rights reserved.

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• Research Support, Non-U.S. Gov't
• Government*
• Health Policy*
• Ill-Housed Persons*
• Intersectoral Collaboration
• North America
• Policy Making*
• Public Health
• Social Determinants of Health*

SYSTEMATIC REVIEW article

The histological and molecular characteristics of early-onset colorectal cancer: a systematic review and meta-analysis.

• 1 School of Medicine and Public Health, Carbone Cancer Center, University of Wisconsin-Madison, Madison, WI, United States
• 2 School of Medicine and Public Health, Department of Population Health Sciences, University of Wisconsin-Madison, Madison, WI, United States

Background: Early-onset colorectal cancer (CRC), defined as diagnosis before age 50, has increased in recent decades. Although more often diagnosed at advanced stage, associations with other histological and molecular markers that impact prognosis and treatment remain to be clarified. We conducted a systematic review and meta-analysis concerning the prevalence of prognostic and predictive tumor markers for early- vs. late-onset CRC, including oncogene mutations, microsatellite instability (MSI), and emerging markers including immune cells and the consensus molecular subtypes.

Methods: We systematically searched PubMed for original research articles published between April 2013–January 2024. Included studies compared the prevalence of tumor markers in early- vs. late-onset CRC. A meta-analysis was completed and summary odds ratios (ORs) with 95% confidence intervals (CIs) were obtained from a random effects model via inverse variance weighting. A sensitivity analysis was completed to restrict the meta-analysis to studies that excluded individuals with Lynch syndrome, a hereditary condition that influences the distribution of tumor markers for early-onset CRC.

Results: In total, 149 articles were identified. Tumors from early-onset CRC are less likely to include mutations in KRAS (OR, 95% CI: 0.91, 0.85-0.98), BRAF (0.63, 0.51-0.78), APC (0.70, 0.58-0.84), and NRAS (0.88, 0.78-1.00) but more likely to include mutations in PTEN (1.68, 1.04-2.73) and TP53 (1.34, 1.24-1.45). After limiting to studies that excluded Lynch syndrome, the associations between early-onset CRC and BRAF (0.77, 0.64-0.92) and APC mutation (0.81, 0.67-0.97) were attenuated, while an inverse association with PIK3CA mutation was also observed (0.88, 0.78-0.99). Early-onset tumors are less likely to develop along the CpG Island Methylator Phenotype pathway (0.24, 0.10-0.57), but more likely to possess adverse histological features including high tumor grade (1.20, 1.15-1.25), and mucinous (1.22, 1.16-1.27) or signet ring histology (2.32, 2.08-2.57). A positive association with MSI status (1.31, 1.11-1.56) was also identified. Associations with immune markers and the consensus molecular subtypes are inconsistent.

Discussion: A lower prevalence of mutations in KRAS and BRAF is consistent with extended survival and superior response to targeted therapies for metastatic disease. Conversely, early-onset CRC is associated with aggressive histological subtypes and TP53 and PTEN mutations, which may serve as therapeutic targets.

1 Introduction

Colorectal cancer (CRC) is the second leading cause of cancer mortality in the United States ( 1 ). The incidence of CRC has steadily declined since the 1980s, largely attributed to greater uptake of colonoscopy screening by adults aged 50 years and older ( 2 ). Concurrently, the incidence of sporadic early-onset CRC, generally defined as CRC diagnosis before age 50 without an underlying hereditary cause, has significantly increased since the mid-1990s ( 2 ). Data from the Surveillance, Epidemiology, and End Results (SEER) program reflect a 2-3% annual increase in the incidence of early-onset CRC ( 3 ). The elevated incidence of early-onset CRC may be explained by birth cohort effects where more recent birth cohorts have increased prevalence of obesity and type 2 diabetes, lower levels of physical activity, and more often consume western-style diets characterized by lower consumption of fruits and vegetables ( 4 ), as well as changes in the composition of the gut microbiome ( 2 ). While early-onset CRC may be caused by hereditary conditions defined by germline mutations in DNA mismatch-repair genes (i.e. Lynch syndrome) or in the tumor suppressor APC (i.e. familial adenomatous polyposis) ( 5 ), these inherited conditions account for a relatively small percentage of early-onset CRC and do not explain the increased prevalence observed in recent decades ( 2 ).

CRC is a heterogeneous disease and the clinicopathological and molecular characteristics of tumors may influence prognosis and response to treatment ( 6 ). Beyond tumor stage, multiple potential prognostic and predictive markers have been identified, including mutations in oncogenes such as KRAS , BRAF , PIK3CA , and TP53 , histological subtypes including mucinous and signet ring carcinomas, and the microsatellite instability (MSI) phenotype ( 7 ). Further, several novel prognostic markers have recently been identified, including immune markers in the tumor microenvironment ( 8 ) and the CRC consensus molecular subtypes ( 9 ). It is anticipated that the continued characterization of molecular phenotypes in CRC will augment traditional clinical markers for therapeutic decision making and support the development of targeted approaches to treatment ( 10 ).

Given the increasing rate of early-onset CRC, recent publications have highlighted potential differences in the clinicopathological and molecular characteristics of tumors based on age of onset ( 11 – 14 ). However, it is currently unclear whether early-onset CRC is distinct from late-onset disease in terms of molecular characteristics and tumor developmental pathways ( 15 ). Understanding the molecular characteristics of early-onset CRC is necessary to guide the development of therapeutic approaches for this condition and to address underlying causes. Therefore, we have completed a systematic review and meta-analysis to comprehensively summarize the evidence linking early-onset CRC to differences in prognostic and predictive tumor markers, including oncogene mutations, histological subtypes, MSI status, as well as anti-tumor immunity and the consensus molecular subtypes.

2.1 Literature review

Articles for this systematic review were identified utilizing a Pubmed search incorporating PRISMA guidelines ( 16 ). Given the wide breadth of the topic and the limited number of relevant articles published prior to 2013, the search was limited to peer-reviewed, original research articles published in English from the last 10 years (April 2013 – April 2023), with relevant keywords and medical subject headings included in the title and/or abstract. The literature review was repeated in January 2024 to identify recently published articles. Specific biomarker terms to include in the literature search were identified from prior reviews, and the search terms “biomark*”, “mark*”, and “character*” were included to capture potentially novel prognostic markers. All search terms included for the literature review are displayed in Supplementary Table S1 . Manuscripts were included that reported the prevalence of prognostic biomarkers in CRC tumors separately for early- vs. late-onset disease. Articles were excluded if the prevalence of tumor clinicopathological or molecular biomarkers were not provided for participants with CRC (see Figure 1 flowchart), or if there was no comparison between early- vs. late-onset CRC (or if the comparison was limited to tumor stage or location only). Articles were also excluded that described hereditary CRC only (e.g. Lynch syndrome), site-specific metastases, or included non-CRC cancers in the analysis samples. For the purposes of this analysis, early-onset disease was defined as CRC diagnosed prior to age 50. To avoid misclassification of early- and late-onset CRC, we excluded papers where late-onset CRC was defined as ≥ 40 years at diagnosis or younger, or where early-onset CRC was defined as ≤ 60 years at diagnosis or older. Lastly, to limit sample overlap where possible, we excluded studies if there was evidence of complete overlap in sample and markers reported with a previously published study, or if a study reported the same outcome in a subsample of a previous study.

Figure 1 Literature review flowchart. a Inappropriate study design includes studies concerning colorectal cancer incidence, colonoscopy or other colorectal cancer screening, population level summary statistics for colorectal cancer, and studies of colorectal cancer in model organisms or in vitro studies. b Markers of interest include oncogene mutations in KRAS , NRAS , BRAF , PIK3CA , PTEN , TP53 , APC , and HER2 ; histological phenotypes including high-grade tumors and mucinous or signet ring histology; molecular carcinogenesis pathways including microsatellite instability and the CpG island methylator phenotype (CIMP); and novel tumor prognostic phenotypes including immune markers in the tumor microenvironment and the consensus molecular subtypes. c Studies where late-onset colorectal cancer was defined as ≥ 40 years at diagnosis (or younger), or early-onset CRC was defined as ≤ 60 years at diagnosis (or older).

The systematic review and meta-analysis was limited to the following markers that have been shown associations with CRC survival and/or therapeutic response in CRC: oncogene mutations in KRAS ( 17 – 20 ), NRAS ( 17 , 21 , 22 ), BRAF ( 17 , 19 , 23 , 24 ), PIK3CA ( 17 , 25 , 26 ), PTEN ( 27 , 28 ), TP53 ( 29 ), APC ( 30 , 31 ), and HER2 amplifications ( 32 – 34 ); histological phenotypes including high-grade tumors ( 35 , 36 ) and mucinous ( 37 , 38 ) or signet ring histology ( 38 , 39 ); molecular carcinogenesis pathways including MSI ( 40 ) and the CpG island methylator phenotype (CIMP) ( 41 ); and novel tumor prognostic phenotypes including immune markers ( 42 , 43 ) in the tumor microenvironment and the consensus molecular subtypes ( 9 , 44 ). Because it is well-established that early-onset CRC is associated with advanced tumor summary stage at diagnosis and rectal tumor location, these markers are not summarized in this review. The literature review was completed by two authors (T.L. and L.P) independently. Disagreements between reviewers were resolved by further review of the manuscript to determine whether the study included a comparison of tumor markers of interest between early- and late-onset CRC. The final decision to include a manuscript was made by the lead author. In total, 1,694 articles were identified from the literature search and 149 were eligible for review ( Figure 1 ). For each study, the potential for bias was evaluated by the lead author using the Newcastle-Ottawa Scale adapted for cross-sectional studies ( 45 ). Pre-registration of the systematic review protocol was not performed.

2.2 Meta-analysis

From each eligible study, the number of mutant and wild-type tumors for each marker in early- and late-onset CRC was extracted by the lead author. Data extraction was completed in duplicate, and the results from the two extractions were compared to identify any errors or inconsistencies in the sample sizes, which were subsequently revised after further review of the original article. If these data were not available from the manuscript, sample sizes were requested from the corresponding author. One study was excluded for which we were unable to obtain the necessary sample sizes from each group ( 46 ). When necessary, sample sizes for separate age groups were combined to create a single category for early-onset and late-onset CRC. For most studies, age 45 or 50 at diagnosis was utilized as the threshold to distinguish early- vs. late-onset CRC, although occasionally other classifications were employed (see Supplementary S2 ). For each study, sample characteristics including overall sample size, country, tumor stage, sex, or other distinguishing features were also extracted. For each marker, an odds ratio (OR) and 95% confidence interval (CI) were calculated using a standard equation ( 47 ). For mutations in oncogenes KRAS, NRAS, BRAF, PIK3CA, PTEN, TP53 , and APC , as well as MSI status and histological subtypes, meta-analyses were completed to compare the prevalence in tumors from early- vs. late-onset CRC. Due to the wide variety of immune markers that have been reported, a meta-analysis was not attempted for the comparison of immune phenotypes in the tumor microenvironment. For each marker that was meta-analyzed, a pooled OR with 95% CI was obtained from a random effects model via inverse variance weighting. The random effects model was selected a priori , as between-study heterogeneity is plausible given variability in the definition of early-onset CRC, as well as differences in tumor location, race, nationality and stage between studies. The random effects meta-analysis is capable of providing unbiased estimates in the presence of heterogeneity and will generally provide more conservative estimates than the fixed-effects model (which assumes no between-study heterogeneity) ( 48 ). Heterogeneity was determined via the Cochrane’s Q statistic and the I 2 statistic. Significant heterogeneity was defined as P <.05 for Cochrane’s Q or I 2 ≥ 50%. To determine whether the meta-analysis estimates were influenced by a single study, a ‘leave-one-out’ sensitivity analysis was conducted for each marker. Because Lynch syndrome may influence the prevalence of tumor markers for individuals with early-onset CRC, a second sensitivity analysis was completed to limit the analysis to studies that specifically excluded individuals with Lynch syndrome or family history of CRC, or that restricted the sample to microsatellite stable tumors. All statistical tests were two-sided, with statistical significance defined using a threshold of P <.05. All meta-analyses were completed using Review Manager 5.4.1 from Cochrane.

In total, 149 articles were reviewed that compared the prevalence of clinicopathological tumor markers in early- vs. late-onset CRC. All meta-analysis results are summarized in Table 1 . Sample characteristics and references for all included studies are presented in Supplementary Table S2 . Results of the bias assessment utilizing the Newcastle-Ottawa Scale are presented in Supplementary Table S4 .

Table 1 Summary of meta-analysis results showing associations between early-onset colorectal cancer and the prevalence of tumor markers, compared to late-onset colorectal cancer.

3.1 Oncogene mutations

The number of studies identified for the following markers is as follows: KRAS mutation ( 49 ); BRAF mutation ( 49 ); NRAS mutation ( 20 ); PIK3CA mutation ( 21 ); PTEN mutation ( 8 ); HER2 amplifications ( 5 ); APC mutation ( 19 ); TP53 mutation ( 20 ). For early-onset CRC, there is evidence for a significantly lower prevalence of mutations in KRAS ( Figure 2 , OR 0.91, 95% CI 0.85-0.98), BRAF ( Figure 3 , OR 0.63, 95% CI 0.51-0.78) and APC ( Figure 4 , OR 0.70, 95% CI 0.58-0.84) compared to late-onset CRC. Early-onset CRC was associated with non-significantly lower prevalence of mutations in NRAS ( Figure 5 , OR 0.88, 95% CI 0.78-1.00, p = .06). Conversely, early-onset CRC is associated with a higher prevalence of mutations in TP53 ( Figure 6 , OR 1.34, 95% CI 1.24-1.45) and PTEN ( Figure 7 , OR 1.68, 95% CI 1.04-2.73). There was no significant difference in the prevalence of PIK3CA mutations ( Supplementary Figure S1 , OR 0.95, 95% CI 0.86-1.05), or HER2 amplifications ( Supplementary Figure S2 , OR 1.64, 95% CI 0.86-3.14). Significant inter-study heterogeneity was observed for mutations in KRAS , BRAF , PTEN , and APC . Hazard ratios for oncogene mutations were stable in the leave-one-out sensitivity analysis ( Supplementary Table S3 ), although the association for NRAS and PTEN mutations did not always reach statistical significance.

Figure 2 Odds ratios for KRAS mutation in early-onset CRC. Data presented as odds ratios (95% confidence interval) for KRAS mutation in early-onset relative to late-onset colorectal cancer. The pooled odds ratio is obtained via a random effects model using inverse variance weighting. AACR, American Association for Cancer Research; MDACC, MD Anderson Cancer Center; MSI, microsatellite instability; MSS, microsatellite stable.

Figure 3 Odds ratios for BRAF mutation in early-onset CRC. Data presented as odds ratios (95% confidence interval) for BRAF mutation in early-onset relative to late-onset colorectal cancer. The pooled odds ratio is obtained via a random effects model using inverse variance weighting. AACR, American Association for Cancer Research; MDACC, MD Anderson Cancer Center; MSI, microsatellite instability; MSS, microsatellite stable.

Figure 4 Odds ratios for APC mutation in early-onset colorectal cancer. Data presented as odds ratios (95% confidence interval) for APC mutation in early-onset relative to late-onset colorectal cancer. The pooled odds ratio is obtained via a random effects model using inverse variance weighting. AACR, American Association for Cancer Research; COH, City of Hope National Medical Center; CI, confidence interval; EO-CRC, early-onset colorectal cancer; MDACC, MD Anderson Cancer Center; MSKCC, Memorial Sloan Kettering Cancer Center; UCD, University of California, Davis.

Figure 5 Odds ratios for NRAS mutation in early-onset colorectal cancer. Data presented as odds ratios (95% confidence interval) for NRAS mutation in early-onset relative to late-onset colorectal cancer. The pooled odds ratio is obtained via a random effects model using inverse variance weighting. AACR, American Association for Cancer Research; CI, confidence interval; EO-CRC, early-onset colorectal cancer; MDACC, MD Anderson Cancer Center.

Figure 6 Odds ratios for TP53 mutation in early-onset colorectal cancer. Data presented as odds ratios (95% confidence interval) for TP53 mutation in early-onset relative to late-onset colorectal cancer. The pooled odds ratio is obtained via a random effects model using inverse variance weighting. AACR, American Association for Cancer Research; CI, confidence interval; EO-CRC, early-onset colorectal cancer; MDACC, MD Anderson Cancer Center; MSI, microsatellite instability; MSS, microsatellite stability.

Figure 7 Odds ratios for PTEN mutation in early-onset colorectal cancer. Data presented as odds ratios (95% confidence interval) for PTEN mutation in early-onset relative to late-onset colorectal cancer. The pooled odds ratio is obtained via a random effects model using inverse variance weighting. AACR, American Association for Cancer Research; CI, confidence interval; EO-CRC, early-onset colorectal cancer; MDACC, MD Anderson Cancer Center.

Fifty studies were identified that specifically excluded individuals with Lynch syndrome or family history of CRC, or that restricted the analysis to individuals with microsatellite stable tumors ( Table 1 ; Supplementary Table S2 ). Compared to the full analysis, the association between early-onset CRC and BRAF (OR 0.77, 95% CI 0.64-0.92) and APC mutations (OR 0.81, 95% CI 0.67-0.97) were attenuated but remained statistically significant, while the associations with KRAS , NRAS , and TP53 mutations were similar. Further, an inverse association between early-onset CRC and PIK3CA mutation was also observed (OR 0.88, 95% CI 0.78-0.99).

3.2 Molecular carcinogenesis pathways

There were 10 studies that compared the prevalence of CIMP-high status in early- vs. late-onset CRC, and 64 studies that compared MSI status. Individuals with early-onset CRC had significantly lower odds for CIMP-high tumors compared to individuals with late-onset disease ( Supplementary Figure S3 , OR 0.24, 0.10-0.57), but significantly higher odds for the MSI phenotype ( Supplementary Figure S4 , OR 1.31, 1.11-1.56). Significant heterogeneity was observed for both markers. Associations were stable in the leave-one-out sensitivity analysis ( Supplementary Table S3 ), and after limiting the analysis to studies that excluded individuals with Lynch syndrome or family history of CRC ( Table 1 ).

3.3 Histological characteristics

There were 86 studies that compared the prevalence of high-grade tumors (i.e. poorly differentiated or undifferentiated tumors) in early- vs. late-onset CRC, 57 studies that compared the prevalence of mucinous histology (or mucinous characteristics), and 44 studies that reported on signet ring cell carcinomas. In early-onset CRC, there was evidence for a significantly higher prevalence of high-grade (i.e., poorly differentiated) tumors ( Supplementary Figure S5 , OR 1.20, 95% CI 1.15-1.25), as well as mucinous tumors ( Supplementary Figure S6 , OR 1.22, 95% CI 1.16-1.27), and signet ring cell carcinomas ( Supplementary Figure S7 , OR 2.32, 2.08-2.57). Significant inter-study heterogeneity was observed for all histological markers. All associations were stable in the leave-one-out sensitivity analysis ( Supplementary Table S3 ) and after limiting the analysis to studies that excluded individuals with Lynch syndrome or family history of CRC ( Table 1 ).

3.4 Immune markers

There have been nine studies to investigate age differences in the immune cell populations of CRC tumors, with inconsistent results ( 49 – 57 ). Du et al. reported that Chinese patients with sporadic early-onset CRC showed significantly higher densities of multiple immune cell populations in the tumor microenvironment compared to patients with late-onset disease, including higher levels of B cells, CD4+ T cells, CD8+ T cells, neutrophils, macrophages, and dendritic cells ( 50 ). By contrast, Ugai et al. reported no significant differences in the populations of T cells, macrophages, and other myeloid cells in participants with early- vs. late-onset CRC from the Nurses’ Health Study and Health Professionals Follow-up Study ( 51 ). In a small study of 14 tumors utilizing single cell RNA sequencing, Li et al. reported that early-onset CRC was associated with lower levels of effector CD8+ T cells and antigen-presentation in the tumor microenvironment, but higher levels of naïve CD8+ T cells and immunosuppressive regulatory T cells compared to individuals with late-onset disease, suggesting an impaired anti-tumor immune response for early-onset CRC ( 54 ). Because MSI status may influence the anti-tumor immune response, recent studies have examined associations between early-onset CRC and tumor lymphocyte populations in samples limited to microsatellite stable tumors, or after careful exclusion of participants with Lynch syndrome ( 56 , 57 ). In a matched analysis of microsatellite stable tumors, Lu et al. (2023) reported that there was no significant differences between early- and late-onset CRC for the infiltration of 22 different lymphocyte populations in the tumor microenvironment ( 57 ). Likewise, Andric et al. found no significant difference for five lymphocyte populations (total T cells, conventional CD4+ and CD8+ T cells, regulatory T cells, and γδ T cells) in a matched sample limited to cases of sporadic CRC ( 56 ). Other studies have reported no significant differences between early and late-onset CRC for the density of total tumor infiltrating lymphocytes ( 53 , 55 ).

3.5 The consensus molecular subtypes

There have been six studies to determine the distribution of consensus molecular subtypes (CMS) for CRC by age at diagnosis ( 50 , 57 – 61 ). Utilizing tumor tissues samples from 626 individuals diagnosed with CRC from The Cancer Genome Atlas and MD Anderson Cancer Center, Willauer reported that the CMS1 subtype was more common among patients aged 30-39 years at diagnosis (46%) compared to older participants, while the CMS4 subtype was less common (13%) ( 58 ). Conversely, in a smaller study from the Nanjing Colorectal Cancer Cohort, Du et al. reported a higher prevalence of the CMS4 subtype in early- vs. late-onset CRC (36.7% vs. 12.2%, respectively), although the comparison between age groups did not reach statistical significance ( 50 ). Recent results, including from a small sample of South Korean participants ( 59 ) and additional analyses of The Cancer Genome Atlas ( 60 , 61 ) did not show any significant association between early-onset tumors and the distribution of consensus molecular subtypes.

4 Discussion

Sporadic early-onset CRC is a significant public health concern, increasing by 2-3% per year in the U.S. since 1990 ( 3 , 62 ). Early-onset CRC is more often diagnosed at advanced stages compared to late-onset disease ( 63 , 64 ). However, there is inconsistent evidence that survival varies between early- and late-onset CRC ( 65 , 66 ), complicated by reports that younger patients receive more aggressive systemic treatment ( 67 – 69 ). Thus, international guidelines do not endorse separate treatment recommendations for early-onset disease ( 70 ). Investigating the associations between early-onset tumors and molecular and histological characteristics, and novel tumor markers including immune cell populations, may help to guide the development of therapies that benefit early-onset CRC. Further, highlighting associations between early-onset CRC and tumor markers may aid in the design of clinical trials for targeted therapies. To the authors’ knowledge, this is the first comprehensive systematic review and meta-analysis of tumor prognostic and predictive markers in early-onset CRC. We found that early-onset CRC was associated with a lower prevalence of oncogene mutations in KRAS , BRAF , NRAS , and APC , but a higher prevalence of TP53 and PTEN mutations and adverse histologic subtypes, with inconsistent associations for immune cell populations and the consensus molecular subtypes.

KRAS , BRAF , and NRAS encode proteins that act downstream of the epidermal growth factor receptor (EGFR) and activate Mek/Erk signaling ( 21 , 71 ). Mutations in these oncogenes are negative predictive markers for EGFR inhibition in metastatic CRC ( 17 , 18 ) and are associated with inferior survival outcomes across tumor stage ( 19 , 20 , 23 , 72 ), including for early-onset CRC ( 73 – 75 ). Early-onset CRC is associated with a lower prevalence of mutations in these genes compared to late-onset disease, indicating that individuals with metastatic early-onset CRC may be more likely to benefit from EGFR inhibition. Notably, the association with NRAS mutations was not statistically significant, which may be due to the scarcity of this marker ( 76 ). Further, the association with BRAF mutation was attenuated but still statistically significant in studies that excluded individuals with Lynch syndrome, who are less likely to have BRAF mutations compared to sporadic disease ( 77 ). Further, this sensitivity analysis revealed an inverse association with PIK3CA mutation, which has also been linked to higher risk for mortality and resistance to EGFR inhibition ( 17 , 78 ). Conversely, early-onset CRC was associated with a higher proportion of mutations in tumor suppressor PTEN , which encodes a lipid-phosphatase that suppresses the activity of PI3k/Akt/mTOR signaling and interacts with the EGFR pathway ( 27 ). Loss of PTEN activity has been linked to resistance to EGFR inhibition in metastatic CRC ( 79 ) but is not currently used in clinical decision making. Pharmaceutical therapies to restore normal PTEN activity are under development but have not been evaluated in CRC. Early-onset CRC was associated with a significantly higher prevalence of TP53 mutations, which cause loss of p53 tumor suppressor activity and pro-tumorigenic gain of function effects that accelerate cell proliferation, angiogenesis, and metastasis ( 80 ). TP53 mutations are found in approximately 60% of tumors and may promote resistance to EGFR inhibitors and chemotherapies that rely on wild type p53 to induce cellular apoptosis (e.g. 5-fluorouracil and Oxaliplatin) ( 29 ). Consequently, targeted therapies to restore wild type p53 activity or degrade mutant p53, or to inhibit downstream effector pathways, are currently being investigated in clinical trials ( 81 ). Potentially, individuals with early-onset CRC may be more likely to benefit from treatments that inhibit pro-tumorigenic p53 activity and should be targeted for enrollment in these trials.

Early-onset CRC was associated with a lower prevalence of APC mutation, a key driver of the canonical adenoma-carcinoma pathway ( 82 ). APC mutations are present in approximately 80% of CRC tumors ( 11 , 12 , 14 ), and recent evidence indicates that APC -mutant tumors are associated with extended overall and progression-free survival compared to wild type ( 30 , 31 ) ( 5 ). Notably, the association with APC mutation was attenuated but still statistically significant when limiting the analysis to studies that excluded individuals with Lynch syndrome, or that included microsatellite stable tumors only. Individuals with early-onset CRC had a higher prevalence of MSI, defined by a high density of somatic mutations in short, non-coding sequences caused by defects in DNA mismatch repair ( 40 ). MSI is associated with lower risk for overall mortality and distant metastases compared to microsatellite stable tumors, including in early-onset CRC ( 75 ). Further, MSI tumors secrete truncated proteins that trigger an anti-tumor immune response ( 83 ), and consequently MSI is a positive predictor for response to immune checkpoint inhibitors ( 83 ). Our findings therefore highlight the importance of MSI testing for individuals younger than 50, in accordance with clinical guidelines ( 70 ). Unexpectedly, the association between early-onset CRC and MSI status was modestly strengthened in studies that excluded individuals with known Lynch syndrome, which causes tumors with MSI ( 84 ). Because a significant proportion of individuals with Lynch syndrome may be unaware of the condition ( 85 ), it is possible that the exclusion of Lynch syndrome was incomplete in some studies. Early-onset CRC was associated with a lower prevalence of the CpG island methylator phenotype (CIMP), characterized by methylation and inactivation of tumor-suppressor genes ( 86 ). Although CIMP has been linked to poor prognosis in multiple studies, it currently has limited value as a prognostic marker due to a lack of standardized assessment and competing effects of MSI and BRAF mutation, which are associated with CIMP ( 41 ).

We also found that early-onset CRC is associated with higher odds for tumors with more aggressive histological features, including poorly differentiated tumors, mucinous carcinomas, and signet ring cell carcinomas ( 38 , 87 ). The association with signet ring features was especially pronounced (OR [95% CI]: 2.32 [2.08-2.57]). Although signet ring carcinomas comprise only 1% of CRC tumors ( 39 ), this feature is present in 2-3% of early-onset tumors. A recent meta-analysis showed that signet ring carcinomas were associated with significantly higher risk for overall mortality and recurrence compared to conventional adenocarcinomas ( 88 ). Results were similar for mucinous tumors, which comprise approximately 10-15% of CRCs ( 89 ). The associations between histological subtypes and colorectal cancer mortality, especially poorly differentiated tumors and signet ring carcinomas, have been validated in early-onset CRC ( 90 – 93 ). Currently, there are no treatments that specifically target mucinous or signet ring cell carcinomas and treatment guidelines do not distinguish between histological subtypes ( 70 ).

The observed associations between early-onset CRC and certain histological and molecular tumor characteristics may be explained in part by differences in tumor location ( 94 ). Approximately 30% of early-onset tumors are located in the rectum, versus 20% of late-onset tumors ( 64 , 95 ). KRAS, BRAF, PIK3CA , and NRAS mutations are enriched in proximal tumors ( 96 , 97 ) while TP53 mutations are enriched in rectal tumors ( 98 ). Notably, studies that were limited to individuals with tumors in the distal colon or rectum have not shown a consistent association between early-onset CRC and the presence of oncogene mutations ( 46 , 55 , 56 , 99 – 102 ). For example, a study with more than 1,000 distal and rectal tumors showed no significant age difference in KRA S, BRAF, NRAS , PIK3CA , TP53 , or APC mutations ( 46 ). Conversely, in a large-scale analysis with detailed stratification by tumor location, Ugai et al. found that early-onset CRC had a lower prevalence of BRAF mutations for all tumor sites except the sigmoid colon and rectum ( 103 ). Notably, aggressive histological subtypes are overrepresented in the proximal colon ( 104 ), and consequently the association with early-onset CRC is not explained by differences in tumor location.

We found inconsistent evidence linking early-onset CRC to differences in ‘novel’ tumor prognostic and predictive markers including populations of immune cells in the tumor microenvironment ( 8 ). A recent meta-analysis demonstrated that a higher density of tumor infiltrating lymphocytes was associated with reduced overall mortality among 20,015 individuals with CRC (HR [95% CI]: 0.65 [0.54-0.77]) ( 42 ), while others have shown that an ‘immunoscore’ encompassing cytotoxic T cells and CD3+ cells was a superior prognostic marker compared to the tumor stage ( 105 , 106 ). Currently, the association between early-onset CRC and the anti-tumor immune response has been inconsistent ( 48 – 50 , 52 , 53 , 55 , 56 , 58 ). Notably, higher rates of MSI in early-onset CRC due to Lynch syndrome may obscure associations with immune markers in sporadic disease, as MSI tumors trigger a robust anti-tumor immune response ( 83 ). Studies limited to microsatellite stable tumors or that carefully excluded participants with hereditary syndromes have tended to show no significant differences in immune cell populations between early- and late-onset CRC ( 51 , 56 , 57 ). Likewise, there is currently no consistent evidence that the distribution of consensus molecular subtypes differs between early- and late-onset CRC, with most studies reporting null findings ( 50 , 57 , 59 – 61 ). The consensus molecular subtypes have shown to be a robust predictor of mortality outcomes independent of tumor stage ( 107 ), but to the authors’ knowledge have not been validated specifically in early-onset CRC. Further, the identification of novel molecular subtypes in early-onset CRC based on tumor gene expression is an area for future research.

Strengths of this study include the comprehensive nature of the search strategy, as we were able to summarize the evidence for age-related differences in the prevalence of established tumor prognostic markers as well as emerging markers including immune cell populations in the tumor microenvironment and the consensus molecular subtypes. Further, the large number of studies identified for most markers allowed for relatively precise estimates of the association with early-onset CRC. Lastly, to better understand the associations between early-onset CRC and tumor markers in sporadic disease, we completed a sensitivity analysis limited to studies that excluded individuals with known Lynch syndrome (or family history of CRC). This analysis is also attended by several limitations. Due to the breadth of the review, our literature search was limited to original research studies published within the last ten years in Pubmed. Consequently, it is possible that a relevant study was missed. However, this is unlikely to be a significant limitation given the paucity of large tumor genomic studies published prior to 2013 and the comprehensive nature of our search strategy. Further, there was evidence for significant heterogeneity in the estimates for most tumor markers, but we were unable to investigate underlying sources of inter-study heterogeneity because the prevalence of tumor prognostic markers was rarely presented in subgroups defined by tumor location, tumor stage, or MSI status. Between-study differences in the definitions of early- and late-onset CRC may also have contributed to heterogeneity, although we excluded studies where misclassification of early-onset CRC was apparent. Lastly, although we attempted to control for bias by performing a sensitivity analysis limited to studies that accounted for Lynch syndrome in the study design, it is possible that residual confounding by hereditary conditions or differences in tumor location may have biased the results.

5 Conclusions

In summary, early-onset CRC was associated with a lower prevalence of mutations in several oncogenes linked to mortality and poor therapeutic response, including KRAS , BRAF , and NRAS compared to individuals with late-onset disease. Conversely, early-onset disease was associated with a higher prevalence of potentially harmful mutations in TP53 and PTEN , as well as aggressive histological subtypes including mucinous and signet ring cell carcinomas. In part, these associations may reflect the higher prevalence of rectal tumors in early-onset CRC and the effect of hereditary syndromes on tumor markers. Given these findings and the alarming rise in the incidence of early-onset CRC, it is essential that clinical trials for targeted therapies enroll sufficient numbers of individuals with early-onset disease to evaluate their efficacy in this subgroup. Additional research is required to clarify the relationships with novel tumor characteristics including immune markers and to identify molecular subtypes specific to early-onset CRC that can inform treatment and prognosis.

Data availability statement

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

Author contributions

TL: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation. LP: Writing – review & editing, Writing – original draft, Investigation, Data curation. SW: Writing – review & editing, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This work was supported by the National Cancer Institute of the National Institutes of Health [NIH/NCI] under grants R00 CA207848 and R01 CA255318.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fonc.2024.1349572/full#supplementary-material

Abbreviations

AACR, American Association for Cancer Research; APC, adenomatous polyposis coli; CI, confidence interval; CIMP, CpG island methylator phenotype; CMS, consensus molecular subtypes; COH, City of Hope (National Medical Center); CRC, colorectal cancer; EGFR, epidermal growth factor receptor; MDACC, MD Anderson Cancer Center; MSI, microsatellite instability; MSKCC, Memorial Sloan Kettering Cancer Center; MSS, microsatellite stable; OR, odds ratio; SEER, Surveillance, Epidemiology, and End Results; TIL, tumor infiltrating lymphocytes.

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Keywords: colorectal cancer, colon cancer, rectal cancer, early-onset, oncogenes, prognosis, molecular characteristics

Citation: Lawler T, Parlato L and Warren Andersen S (2024) The histological and molecular characteristics of early-onset colorectal cancer: a systematic review and meta-analysis. Front. Oncol. 14:1349572. doi: 10.3389/fonc.2024.1349572

Received: 04 December 2023; Accepted: 16 April 2024; Published: 26 April 2024.

Reviewed by:

Copyright © 2024 Lawler, Parlato and Warren Andersen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Shaneda Warren Andersen, [email protected]

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

• Open access
• Published: 26 April 2024

A Systematic review of the factors that affect soccer players’ short-passing ability—based on the Loughborough Soccer Passing Test

• Bihan Wang 1   na1 ,
• Bin Wan 1   na1 ,
• Shu Chen 1 ,
• Yu Zhang 1 ,
• Xiaorong Bai 2 ,
• Wensheng Xiao 2 ,
• Changfa Tang 1 &
• Bo Long 1  

BMC Sports Science, Medicine and Rehabilitation volume  16 , Article number:  96 ( 2024 ) Cite this article

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Metrics details

This study synthesizes evidence from the Loughborough Passing Test to evaluate the short-passing ability of soccer players and summarizes the reported variables that affect this ability to provide support for the development and improvement of short-passing abilities in soccer players.

In this systematic review using the PRISMA guidelines, a comprehensive search was conducted in Web of Science, PubMed, and EBSCOhost from inception to July 2023 to identify relevant articles from the accessible literature. Only studies that used the Loughborough test to assess athletes' short-passing ability were included. The quality of the included studies was independently assessed by two reviewers using the PEDro scale, and two authors independently completed the data extraction.

Based on the type of intervention or influencing factor, ten studies investigated training, nine studies investigated fatigue, nine studies investigated supplement intake, and five studies investigated other factors.

Evidence indicates that fitness training, small-sided games training, and warm-up training have positive effects on athletes' short-passing ability, high-intensity special-position training and water intake have no discernible impact, mental and muscular exhaustion have a significantly negative effect, and the effect of nutritional ergogenic aid intake is not yet clear. Future research should examine more elements that can affect soccer players' short-passing ability.

Trial registration

https://inplasy.com/ ., identifier: INPLASY20237.

Peer Review reports

Introduction

Soccer is a game of skills and strategy, and one of the most crucial techniques is short passing [ 1 , 2 , 3 ]. A player's ability to make short passes is important for the team to initiate offense and control the pace of the game. Soccer players can more effectively control the game by strategically use their short passing ability. Making multiple quick, short passes in succession can speed up the game, complete the attacking strategy, and increase pressure on the defence of the opposition, which can provide scoring opportunities [ 4 ]. According to a study, most goals are preceded by short passes [ 5 ].

Players who use short-passing techniques in the game must decide on the pass's timing, strength, and direction under time and space constraints based on the placement of teammates and opponents on the field. However, the conventional short-passing ability assessment employs a single short-passing ability test. The most striking feature of this type of test is that it is performed in a relatively static environment with a short pass to a target or teammate at a known distance and direction; therefore, only motion patterns are shown throughout the test, and it has limited ecological validity [ 6 , 7 , 8 , 9 ]. This type of test cannot be used to effectively evaluate the short-pass technique of athletes with different levels of competitive ability [ 10 , 11 , 12 ]. In contrast to conventional short-passing ability tests, the Loughborough Soccer Passing Test (LSPT), as shown in Fig.  1 , as a multitask test, has advantages in the evaluation of athletes' short passes: it requires participants to remember the relative orientation of the target, process oral information for quick decision-making, squelch potential errors, and make flexible cognitive transitions while using their short-passing ability. As a result, the LSPT is consistent with the shifting circumstances of soccer matches [ 12 , 13 ]. The LSPT procedure is manageable and requires subjects to pass the ball 16 times while surrounded by a rectangular bench. Each bench has a colourful metal strip or coloured cardboard (0.6 × 0.3 m) that can be utilized as a target area to make an effective pass in one of four randomly selected colour sequences. Subjects must complete 16 brief passes of the test as quickly and accurately as they can. Time-related metrics are used to define LSPT scores, including execution time (the amount of time needed to complete 16 passes), penalty time (the amount of time added for mistakes, including incorrect passes and sluggish performance), and total time (execution time plus penalty time). All LSPT time values are inversely correlated with a player's short-passing ability in soccer (a player with a lower LSPT time value has greater short-passing ability). The LSPT is currently used in research on athlete selection in Australia [ 14 ], the Netherlands [ 15 ], and France [ 16 ]. Several studies have shown that the LSPT has good retest reliability and good discriminant validity for players of different sport levels, ages, and genders [ 12 , 17 ].

Layout of the LSPT [ 12 , 18 ]

Regrettably, despite its importance, there is no systematic review of short-passing abilities or the factors that influence them. Systematic reviews of soccer skills have been conducted on most or all skills or overall athletic performance [ 19 , 20 , 21 ], but there is a lack of systematic reviews of specific soccer skills. Due to the importance of short-passing abilities, there is a great need for a more comprehensive analysis of the research on soccer players' short-passing abilities to statistically synthesize the various findings and to examine the factors that affect soccer players' short-passing abilities. The purpose of this paper is to review and analyse research on the factors that affect the short-passing ability of soccer players to contribute to improvements in soccer players’ short-passing ability.

This systematic review used the PRISMA guidelines [ 22 ] and was registered in the International Platform for Registered Programs for Systematic Reviews and Meta-Analyses (INPLASY); https://inplasy.com , INPLASY202370041.

Search strategy

A comprehensive, electronic search of the literature was conducted without data restrictions in Web of Science, PubMed, and EBSCOhost on July 10, 2023, using a search strategy developed by two authors (WBH and XWS). The keyword combinations used were: (("Pass" OR "Skill" OR "Technology" OR "Technique" OR "Art" OR "Performance" OR "Ability" OR "Capacity") AND ("Soccer" OR "Football") AND ("LSPT" OR "Loughborough Soccer Passing Test")). Additionally, the researchers explored Google Scholar and the reference lists of the included studies for potential papers that could meet the inclusion criteria for additional related citations.

Eligibility criteria

The overall, intervention, comparison, outcome, and study design (PICOS) criteria were the inclusion criteria for this study, as detailed in Table 1 . Studies were included if they met the following requirements: 1. football players were the subjects of the study; 2. the paper must include at least one study that aimed to assess the effect of a factor or an intervention on the short-passing ability of soccer players; 3. the method used to assess the short-passing ability of the subjects of the study must have been LSPT. Regardless of the factor that influences a soccer player's short-passing ability, any study that met the above three requirements was included in this systematic review.

Studies that met the following criteria were excluded: 1. conferences, overviews, newsletters, book reviews, and studies that were not supported by data and were not analysed statistically; 2. studies that did not quantitatively evaluate the short-passing abilities of the subjects or evaluated them without using the LSPT; 3. studies that did not apply to the vast majority of soccer players, such as the effect of a particular religious practice on the short-passing ability of a soccer player of that religion or the effect of a certain factor on the short-passing ability of a soccer player with a disability.

Study selection

The following procedure was used to choose the papers. First, prior to importing the studies into EndNote X9 to check for duplication, an experienced librarian assisted with the search strategy by putting key phrases into the three major databases to search for articles. Second, to find pertinent research, two independent reviewers (WBH and XWS) examined the titles and abstracts of all identified papers in accordance with the inclusion and exclusion criteria of the study.

Data extraction

Two independent reviewers (WBH and XWS) completed the data extraction. Any disputes were explored further. When necessary, a third reviewer (BXR) participated until consensus was reached. The records included (1) the author and year of publication; (2) the study design; (3) participant characteristics, namely, age, sex, and athletic level; (4) the characteristics of the intervention; and (5) the final research outcomes.

Quality assessment

Two authors (WBH and XWS) independently utilized the PEDro scale, with disagreements resolved by a third rater (BXR). The eligibility criteria in the scale were not included in the total score, as they were related to external validity. The total PEDro score ranges from 0 to 10. The higher the score, the better the methodological quality. A score of 8 to 10 indicates a study of excellent methodological quality, 5 to 7 is considered to indicate good quality, 3 to 4 is considered a study of average quality, and values less than 3 points are considered to indicate fair quality. A score lower than 3 is considered a poor-quality study [ 23 ].

As shown in Fig. 2 , the electronic search of the relevant databases yielded 147 potentially relevant articles (54 from Web of Science, 30 from PubMed, and 63 from EBSCOhost), while an additional five studies were found through Google Scholar and references. The titles and abstracts of 65 publications were evaluated for conformity after duplicates were eliminated ( n = 87). After 17 items were deleted at the title and abstract levels, the remaining 48 articles were read. Following this reading, an additional 15 publications were excluded, and 33 studies that met all the inclusion criteria for the systematic review were retained. The characteristics of the included studies are detailed in Table 2 .

PRISMA flow chart of the study selection process

Demographic characteristics

The pertinent details of the studies are presented in Table 2 . The age of the players ranged from 8 to 25.5 ± 5.2 years. With regard to the players’ gender, most of the studies reported male players, two studies examined female players [ 41 , 42 ], and only one study reported on male and female players [ 52 ].

Intervention characteristics

For ease of generalization and induction, other factors included motivation, soccer field, verbal interaction, visual observation, and salbutamol intake; these are presented in Fig. 3 . Of the 33 included studies, 24 were one-time intervention studies and 9 were long-term intervention studies. Interventions/influencing factors included training ( n = 10), fatigue ( n = 9), supplement intake ( n = 9), and other factors ( n = 5). The ten papers on the influence of training on football players' short-passing ability included fitness training ( n = 4), small-sided games training ( n = 2), warm-up training ( n = 3) and high-intensity position-specific training ( n = 1). The 9 papers on the effect of fatigue on short-passing ability in soccer players included mental fatigue ( n = 5) and muscle fatigue ( n = 4). The 9 papers on the effect of supplement intake on short-passing ability in soccer players included water intake ( n = 2) and nutritional ergogenic aid intake ( n = 7). The five papers on other factors that influence football players' short-passing ability included motivation ( n = 1), verbal interaction ( n = 1), football field ( n = 1), visual observation ( n = 1) and salbutamol intake ( n = 1).

Influence Factor Chart of Short-passing Skill of Soccer Players

Among the studies ( n = 10) on the effects of training on short-passing ability in soccer players, with the exception of two one-time intervention studies [ 30 , 32 ], 1) all studies explicitly reported the total duration of the intervention, with the shortest being 5 days [ 31 ] and the longest being 22 weeks [ 25 ]; 2) most of the studies explicitly reported the duration of each intervention, with the shortest being 16 minutes [ 33 ] and the longest being 98 minutes [ 25 ] and only two studies failing to explicitly report the duration of each intervention [ 27 , 31 ]; 3) all studies explicitly reported the frequency of intervention, which was once a day in one study [ 31 ], twice a week in three studies [ 28 , 29 , 33 ], three times a week in two studies [ 26 , 27 ], between 2 times a week and 4 times a week in one study [ 25 ], and 2 times in the first week and 3 times in weeks 2 to 4 in one study [ 24 ]. In the studies in which fatigue affected the short-passing ability of soccer players ( n = 9), all reported in detail the intervention protocols used. In terms of mental fatigue, four studies used the Stroop task [ 18 , 34 , 35 , 37 ], one study used Brain It On software [ 36 ], one study used the LSPT random order and clockwise order tasks in addition to the Stroop task [ 18 ], and four studies performed muscle training [ 38 ], soccer matches [ 39 ], high-intensity interval training [ 40 ], and resistance training [ 41 ]. In studies on the effects of supplement intake on short-passing ability in soccer players ( n = 9), 1) all studies reported the intake dose of supplements; 2) all studies explicitly reported the type of supplement ingested, including water, carbohydrate solution, caffeine solution, and carbohydrate caffeine solution (i.e., carbohydrate solution mixed with caffeine solution). In two studies only water was ingested [ 42 , 43 ], in three studies only carbohydrate solutions were ingested [ 44 , 45 , 48 ], two studies used only caffeine solution [ 46 , 50 ], carbohydrate solutions and carbohydrate caffeine solutions were ingested in one study [ 47 ], and carbohydrate solution, caffeine solution, and carbohydrate caffeine solution were ingested in one study [ 49 ]. All other studies of factors that affect short-passing ability in soccer players ( n = 5) provided a clear description or explicit definition of the substance or method of intervention.

Study quality assessment

The quality of the studies is presented in Table 3 . The PEDro checklist was used to assess the quality of the included studies. The results showed that eight studies received a score of 3 or 4, indicating average quality, and 18 studies scored 5 to 7 points, which was considered good quality. Moreover, seven studies had scores ranging from 8 to 10 points and were considered to have excellent methodological quality.

Outcome and measures

The results of the current study were divided into groups based on the various interventions and influencing factors that were found to have an impact on soccer players' short-passing ability.

The effect of training on the short-passing ability of soccer players

Fitness training.

Four studies examined the impact of fitness training on soccer players' short-passing abilities [ 24 , 25 , 26 , 27 ]. The fitness training methods included aerobic interval training [ 24 ], skill combined with agility training [ 25 ], balance training [ 26 ], and strength combined with endurance training [ 27 ]. The subjects included amateur players [ 26 ], youth players [ 24 ], players with five years of experience [ 27 ], and regional sub-elite players [ 25 ]. The results of these studies demonstrate that fitness training improves soccer players' short-passing abilities and is more effective than the training methods used in the control groups of the respective studies.

Small-sided games training

This review comprises two studies that examined the impact of small-sided games training on soccer players' short-passing abilities [ 28 , 29 ]. The participants included amateur players [ 29 ] and professional players [ 28 ]. Both studies found that small-field match training improved short-passing ability in soccer players and demonstrated that small-field match training was more effective than repetitive sprint training and conventional aerobic interval training, respectively, which were used by their control groups.

Warm-up training

The influence of warm-up training on soccer players' short-passing abilities was examined in three studies [ 30 , 31 , 32 ]. One of these studies examined pre-match warm-up training, while the other two explored halftime rewarm-up training. These studies used four warm-up training methods, including passing warm-up training [ 30 ], foam axle rolling training [ 32 ], leg press training, and small-sided games training [ 30 ]. The participants included non-elite players [ 31 ] and professional players [ 30 , 32 ]. Of the four training methods, foam axle rolling training [ 32 ] and leg press training [ 30 ] performed during halftime did not significantly affect players’ short-passing ability, while the remaining two warm-up training methods positively affected players’ short-passing ability [ 30 , 31 ].

High-intensity special-position training

Only one study included in this systematic review presented inferences about the effect of high-intensity special position training on soccer players' short-passing abilities [ 33 ]. The participants in this study were national youth events and professional soccer training services. This study revealed no improvement in short-passing ability after high-intensity special-position training [ 33 ].

The effect of fatigue on the short-passing ability of soccer players

Mental fatigue.

This review included five studies that examined the impact of mental fatigue on soccer players' short-passing abilities [ 18 , 34 , 35 , 36 , 37 ]. The participants included youth players [ 36 ], trained players [ 18 ], players competing at the national level [ 37 ], and professional players [ 34 , 35 ]. These five studies revealed a significant negative impact of mental weariness on soccer players' short-passing abilities.

Muscle fatigue

This systematic review comprised four studies that examined how soccer players' short-passing abilities were affected by muscular exhaustion [ 38 , 39 , 40 , 41 ]. Importantly, two of the studies provided indirect confirmation rather than directly investigating how muscular exhaustion affects soccer players' short-passing abilities [ 40 , 41 ]. The participants included college soccer players [ 38 ], high-level competition players [ 40 ], professional elites, sub-elite players [ 41 ], and professional footballers [ 39 ]. These four studies demonstrated that muscle exhaustion can significantly impair soccer players' short-passing abilities.

The effect of supplement intake on short-passing ability in soccer players

Water intake.

This review included two trials that examined the impact of water intake on soccer players' short-passing abilities [ 42 , 43 ]. The participants included semi-professional players [ 43 ] and professional players [ 42 ]. The intake of water had no discernible impact on players' ability to produce short passes in both experiments.

Nutrition ergogenic aid intake

This review comprised seven trials to confirm the impact of nutrition ergogenic aid intake use on football players' short passing ability [ 44 , 45 , 46 , 47 , 48 , 49 , 50 ]. The subjects included semi-professional players, ex-professional players or players who had reached at least college 1st/2nd team standards [ 44 ], semi-professional or non-professional players from college teams [ 45 ], regional top league players [ 46 ], class players [ 47 ], college players [ 48 , 49 ], and casual players [ 50 ]. Only two studies reported a significant positive effect on players' short-passing ability when they ingested a carbohydrate solution [ 48 ] or a caffeine solution [ 46 ]. The results of the remaining five studies indicated that the ingestion of a carbohydrate solution, a caffeine solution, or a carbohydrate caffeine solution did not have a significant effect on players' short-passing ability [ 44 , 45 , 47 , 49 , 50 ].

This review included five studies that examined additional variables that influenced soccer players' short-passing abilities [ 51 , 52 , 53 , 54 , 55 ]. The participants included amateur players [ 51 ], amateur student players [ 52 ], top players in school soccer games [ 53 ], soccer academy high-level players, soccer academy low-level players [ 54 ] and professional junior soccer players [ 55 ]. In five studies, motivation [ 51 ] and verbal interaction [ 52 ] were reported to positively influence players' short-passing ability. O’Meagher et al. (2022) reported no significant difference in players' short-passing ability between grass and artificial turf. One study reported the important effect of visual observation on players' short-passing ability [ 54 ]. Another study showed that salbutamol intake did not have a significant effect on players' short-passing ability [ 55 ].

The growth and performance of soccer players’ technical and tactical skills depend on their level of fitness. The four studies that examined how short-passing abilities in soccer players were affected by fitness training all concluded that players could benefit from the training techniques used in their studies, which included aerobic interval training [ 24 ], strength and endurance training [ 27 ], skill and agility training [ 25 ], and balance training [ 26 ]. This means that a player's short-passing ability benefits not only from technical training but also from fitness training. The training techniques employed in these studies involve only a portion of the fitness training approach, including endurance training, balance training, and strength training. Some studies support the findings of earlier research that showed that fitness training can enhance athletes' abilities [ 56 , 57 , 58 ]. In fact, the same rationale that supports the positive effects of fitness training on athletes' specialized skills in other sports likely applies to the short-passing abilities of soccer players. In addition to athletes’ mastery of the technique itself, athletes’ physical attributes are crucial to the use of the skill. For instance, an athlete's balance directly influences the mass of the short passing, which is a dynamic unilateral technical movement [ 59 , 60 ], especially when a game-time physical altercation with the opponent occurs. Future studies should examine the effects of various fitness training programmes on soccer players' short-passing abilities.

Soccer training for small-sided games is referred to as skill-based match training [ 61 ] or match-based training [ 62 ] and is typically played on a smaller pitch. According to the two included studies on the impact of small-sided games training on soccer players' short-passing ability [ 28 , 29 ], small-sided games training considerably enhances players' short-passing ability. Small-sided games training simulates the athletic demands, physiological intensity, and technical requirements of a soccer game. Compared with traditional short-passing practice (e.g., one-on-one passing, multiple passes to each other), small-sided games training forces players to use short passes more frequently under increased defensive pressure and reduced field size due to the limitations of the rules. In other words, small-sided games training allows players more opportunities to use and practice short passes under time and space pressure [ 62 , 63 ]. This may also explain why small-sided games training improves soccer players' short-passing ability more significantly than traditional short-passing training or other training methods. This means that coaches and players can use small-sided games training drills to improve short passes in real scenarios that are more similar to games.

Soccer training before a game is essential. In recent years, researchers have examined various warm-up training strategies, such as rewarming up during the game's halftime break and conventional pregame warm-up training, as the methods and means of warm-up training have become more varied. In comparison to a ball size of five, Burcak's (2015) study found that pre-match warm-up training with a ball size of four had a positive effect on players' short-passing ability. In a randomized crossover experiment, Zois et al. (2013) discovered that practising for a small-sided game during halftime increased players' short-passing ability. In contrast, the halftime leg press drill had little impact on players' short-passing ability. Similarly, randomized crossover research by Kaya et al. (2021) revealed that halftime foam-axis rolling drills had no positive impact on players' short-passing ability [ 64 , 65 ]. Nevertheless, due to the lower intensity, foam-axis rolling training and leg press training during halftime tend to reduce muscle temperature in athletes who have recently concluded a game's first half. Based on these findings, athletes may decide to maintain their muscular temperature by engaging in rewarming exercises during halftime. However, it is crucial to remember that each player must be evaluated individually. If a player is extremely exhausted at halftime, rewarming up for training may worsen his or her short passing ability and athletic performance.

Soccer players who engage in high-intensity position-specific training practise the skill most pertinent to their position at a high level (90% HRmax) [ 33 ]. Compared to the impact of small-sided games training on soccer players' short-passing abilities, this produces the opposite outcome. Due to the limitations of the field size, small-sided games training may offer more possibilities for practising short-passing techniques. High-intensity position-specific training, in contrast, includes many additional elements and requires less time to improve short-passing ability. This indicates that high-intensity special-position training is used by coaches and players to enhance short passing, which is an unwise choice.

According to one definition, mental tiredness is a psychobiological condition marked by feelings of exhaustion that can occur during or after prolonged periods of perceived exertion [ 66 , 67 ]. The five studies in this paper on the effect of psychological exhaustion on soccer players' short-passing abilities all concluded that psychological exhaustion may be detrimental to these abilities [ 18 , 34 , 35 , 36 , 37 ]. This suggests that coaches and players should pay increased attention to this easily overlooked factor that affects short-passing ability. According to Filipas et al. (2021), mental fatigue has a significant negative impact on U18 players' short-passing abilities as well as a negative, albeit nonsignificant, impact on U14 and U16 players' short-passing abilities; total LSPT times are 7.4% (U14) and 4.2% (U16) greater than the control group. Smith et al. (2017) also performed more thorough statistical analyses using the LSPT penalty time rule. In contrast to players who are not mentally weary, mentally fatigued players targeted errors substantially more and completed passes significantly less frequently. These findings confirm earlier studies suggesting that mental weariness impairs athletes' performance [ 66 , 68 , 69 ]. According to some research, mental weariness can impair a player's ability to concentrate, lengthen reaction times for cognitive activities, and increase a player's risk of making mistakes when using short-passing techniques [ 70 ]. When using short-passing abilities in soccer, players must maintain a high degree of focus and accurate perception to allow them to make the right choices in a highly dynamic environment and under time and space pressures. As a result, athletes should try to prevent developing premature mental tiredness. Cognitive tasks that require considerable energy typically lead to mental weariness [ 71 ]. Therefore, to avoid premature mental fatigue, players should be wary of high levels of pregame cognitive demands (e.g., excessive use of cell phones, tablets, and video games, as well as prolonged cognitive skill training).

Short inter-match recovery times (halftime) and high neuromuscular demands during soccer matches may result in muscle fatigue during the game, decreasing players' abilities and fitness, which may have an impact on match performance [ 72 ]. Researchers of soccer have paid close attention to the impact of muscular fatigue on short-passing ability, one of the skills most often employed by players in games. Two studies directly reported significant negative effects of muscle fatigue on soccer players' short-passing ability [ 38 , 39 ], and two other studies provided indirect support that short-passing ability can have significant negative effects on soccer players. After high-intensity interval training, Draganidis et al. (2013) reported that professional sub-elite players' short-passing abilities deteriorated, and Lyons et al. (2021) found that high-level players' short-passing abilities deteriorated. These findings provide circumstantial evidence that players with short-passing abilities can suffer from muscle exhaustion resulting from persistent dynamic exercise and resistance training [ 73 , 74 ].

These results are consistent with those from earlier investigations. In fact, numerous studies have documented losses in athletic ability and performance that occur as players approach a state of muscular tiredness [ 75 , 76 ], and one study reported that after fatigue training, a considerable drop occurred in shooting scores [ 77 ]. Soccer players’ short-passing abilities might suffer from muscle exhaustion, perhaps as a result of a reduction in muscle functioning capacity [ 78 , 79 ], which decreases the stability and accuracy of a player's passes. The decrease in players' short-passing ability caused by completing short bursts of high-intensity activity at the same absolute workload is also related to players' physical quality [ 39 ]. Therefore, in actual daily training, to prevent premature muscle fatigue from impairing short-passing ability in play, players should enhance their physical training and practice.

The effect of supplement intake on the short-passing ability of soccer players

During a game, soccer players exert both mental and physical effort. Under extreme physical and mental strain, the body is susceptible to water loss and mental exhaustion. Reduced endurance and cognitive function can result from dehydration in athletes with up to 2% body weight loss during exercise [ 80 , 81 ]. Two variables may cause a player's ability to pass short passes to gradually deteriorate throughout sports: dehydration and inadequate water intake. To keep players' short-passing ability or slow down its decline, several studies have tried feeding them a specific volume of water. However, neither of the experiments presented in this study revealed a significant impact of water intake on players' short-passing abilities [ 42 , 43 ]. These findings suggest that soccer players cannot rely on drinking water during a game to prevent a decrease in short-passing ability.

Nutritional ergogenic aid intake

The nutritional ergogenic aid intake is anything that enhances athletic performance. It can be a nutrient, a nutrient metabolite, a food extract (from a plant), or something that is typically present in other items (i.e., caffeine or carbohydrates) [ 82 ]. Carbohydrate solution, caffeine solution, and carbohydrate and caffeine solution were utilized in the seven studies that examined the impact of the nutritional ergogenic aid intake on soccer players' short-passing abilities. O'Reilly et al. (2013) and Foskett et al. (2009) reported that ingesting a carbohydrate solution or a caffeine solution significantly improved players' short-passing ability. The other five studies found no evidence that ingesting a carbohydrate solution, a caffeine solution, or a carbohydrate and caffeine solution significantly improved players' short-passing abilities [ 44 , 45 , 47 , 49 , 50 ]. Three studies indicate that players' short pass ability is positively impacted by nutritional ergogenic supplement intake, but these findings also indicate that this relationship is not statistically significant [ 44 , 45 , 50 ]. Therefore, the impact of nutritional ergogenic aid intake on soccer players' short-passing abilities is unclear and requires additional explanation. In fact, the effects of nutritional ergogenic aid intake on athletes' skills have been similarly ambiguous in other investigations. For instance, Stuart et al. (2005) [ 83 ] reported that rugby players who swallowed a caffeine solution had a 10% increase in passing accuracy on the exam. However, rugby players took the same dose of caffeine solution in the study by Assi and Bottoms (2014), and the findings revealed no appreciable impact on test-passing accuracy [ 84 ]. Belenky et al. (2005) [ 85 ] claimed that ingesting a caffeinated solution enhanced shooting ability, although other studies have demonstrated that doing so did not significantly enhance this ability [ 86 , 87 ]. Therefore, football players are advised to not employ nutritional ergogenic aid intake to maintain their short-passing abilities or to halt their decline. Future research should confirm these findings with additional randomized, double-blind crossover experiments. Future research is necessary to determine the potential impact of other commonly used nutritional ergogenic aids, such as creatine, L-carnitine, protein, and amino acid supplements, on football players' short pass ability.

Other factors affecting short-passing ability in soccer players

The five studies that were evaluated in this section of the paper examined five underappreciated or overlooked factors that may affect soccer players' short-passing abilities [ 51 , 52 , 53 , 54 , 55 ]. Barte et al. (2019) reported that various methods of motivating worn-out players while at rest improve athletes' short-passing abilities. This suggests that motivating players makes practical sense for improving short passes, which provides support for coaches who are accustomed to motivating players. Khalifa et al. (2020) suggested that talking to teammates during halftime can improve short-passing abilities (10.2% reduction in overall LSPT time) and can outperform passive rest (4.2% reduction in total LSPT time).

Players' short-passing abilities did not differ significantly between grass and artificial turf, according to research by Meagher et al. (2022) Players should therefore not worry about the effect that being on two different types of turf may have on their short-passing ability. However, indoor springy wood flooring considerably improved players' short-passing abilities over grass and synthetic turf. Nevertheless, indoor 5-a-side soccer games are usually the only tournaments played on indoor resilient wood floors. Visual observation, as shown by Vansteenkiste et al. (2022), has a significant impact on players' short-passing ability. When players utilize short-passing abilities, spending too much time focusing on the ball might prevent them from seeing teammates' and defenders' locations, which can cause them to miss the ideal opportunity to pass the ball or lose possession. This suggests that players should be more observant of the ever-changing conditions on the field rather than just staring at the ball in the ratios. Coaches must be aware of this key point, which can be easily overlooked, and remind players of it during training, and players must recognize it themselves. Additionally, a study revealed that taking salbutamol had no discernible impact on players' short-passing abilities [ 55 ].

Limitations

This study systematically evaluated the factors that affect soccer players' short-passing ability. The results showed that these factors can be divided into positive and negative categories. This study provides a reference and support for soccer coaches and players to improve their short-passing abilities. However, there are a few limitations to this review. 1) Papers in languages other than English were excluded from the study, which influenced the selection of papers. 2) Because it is unknown whether the participants' sex, age, and level of sport affected the intervention effects of some research, the pertinent conclusions should not be extended without due care. 3) Despite the advantages of the LSPT over the conventional short-passing ability test, the LSPT cannot accurately imitate the intricacies of soccer players' use of the short-passing technique in games. 4) The present results should be applied with caution due to the lack of research on some of the influencing elements included in this study, which could affect the accuracy of some of the conclusions. Nevertheless, we believe that the current study can aid in the development and improvement of short-passing abilities in soccer because it examines some relevant strategies and elements.

Conclusions

This study's findings indicate that a variety of factors can influence soccer players' short-passing abilities. For example, in terms of the effect of training on football players' short-passing abilities, fitness training, small-sided games training, and some warm-up training positively impact these abilities, while high-intensity special-position training has no discernible impact. Mental and muscular exhaustion have a significantly negative effect. In terms of the effect of supplemental intake on football players’ short-passing ability, water intake has no significant effect, and the effect of nutritional ergogenic aid intake is not yet clear. Based on these findings, additional research is encouraged to investigate techniques or variables that affect short-passing ability in soccer players, such as additional training methods (e.g., Specialized short-passing ability training and functional training) and players' own factors (e.g., sleep and mood). However, whether the results of this study apply to all soccer players of all ages, sexes, and athletic levels is unknown. Future research should focus on determining whether a specific subset of the findings is appropriate for a particular group of soccer players. In addition, this study offers only a general directional reference for the sustainable development and improvement of soccer players' short-passing ability.

Availability of data and materials

Data are available on request to the corresponding author by e-mail ([email protected] OR [email protected] OR [email protected]), and registered in the International Platform for Registered Programs for Systematic Reviews and Meta-Analyses (INPLASY); https://inplasy.com , INPLASY202370041.

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A grant from the Ministry of Education of China, Grant No. 20YJA890002, supported this study.

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Bihan Wang and Bin Wan contributed equally to this study.

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College of Physical Education, Hunan Normal University, Changsha, 410006, China

Bihan Wang, Bin Wan, Shu Chen, Yu Zhang, Changfa Tang & Bo Long

School of Physical Education, Huzhou University, Huzhou, 313000, China

Xiaorong Bai & Wensheng Xiao

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Wang, B., Wan, B., Chen, S. et al. A Systematic review of the factors that affect soccer players’ short-passing ability—based on the Loughborough Soccer Passing Test. BMC Sports Sci Med Rehabil 16 , 96 (2024). https://doi.org/10.1186/s13102-024-00880-y

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• Best practices for the dissemination and implementation of neuromuscular training injury prevention warm-ups in youth team sport: a systematic review
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• http://orcid.org/0009-0005-0529-0398 Destiny Lutz 1 ,
• http://orcid.org/0000-0001-6429-4333 Carla van den Berg 1 ,
• http://orcid.org/0000-0003-3056-8169 Anu M Räisänen 1 , 2 ,
• Isla J Shill 1 , 3 ,
• Jemma Kim 4 , 5 ,
• Kenzie Vaandering 1 ,
• Alix Hayden 6 ,
• http://orcid.org/0000-0002-0427-2877 Kati Pasanen 1 , 7 , 8 , 9 ,
• http://orcid.org/0000-0002-5951-5899 Kathryn J Schneider 1 , 3 , 8 , 9 , 10 ,
• http://orcid.org/0000-0002-9499-6691 Carolyn A Emery 1 , 3 , 8 , 9 , 11 , 12 , 13 ,
• http://orcid.org/0000-0002-5984-9821 Oluwatoyosi B A Owoeye 1 , 4
• 1 Sport Injury Prevention Research Centre, Faculty of Kinesiology , University of Calgary , Calgary , Alberta , Canada
• 2 Department of Physical Therapy Education - Oregon , Western University of Health Sciences College of Health Sciences - Northwest , Lebanon , Oregon , USA
• 3 Hotchkiss Brain Institute , University of Calgary , Calgary , Alberta , Canada
• 4 Department of Physical Therapy & Athletic Training , Doisy College of Health Sciences, Saint Louis University , Saint Louis , Missouri , USA
• 5 Interdisciplinary Program in Biomechanics and Movement Science , University of Delaware College of Health Sciences , Newark , Delaware , USA
• 6 Libraries and Cultural Resources , University of Calgary , Calgary , Alberta , Canada
• 7 Tampere Research Center for Sports Medicine , Ukk Instituutti , Tampere , Finland
• 8 McCaig Institute for Bone and Joint Health , University of Calgary , Calgary , Alberta , Canada
• 9 Alberta Chilrden's Hopsital Research Institute , University of Calgary , Calgary , Alberta , Canada
• 10 Sport Medicine Centre , University of Calgary , Calgary , Alberta , Canada
• 11 O'Brien Institute for Public Health , University of Calgary , Calgary , Alberta , Canada
• 12 Department of Community Health Sciences , Cumming School of Medicine, University of Calgary , Calgary , Alberta , Canada
• 13 Department of Paediatrics , Cumming School of Medicine, University of Calgary , Calgary , Alberta , Canada
• Correspondence to Ms Destiny Lutz, Sport Injury Prevention Research Centre, Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada; destiny.lutz{at}ucalgary.ca

Objective To evaluate best practices for neuromuscular training (NMT) injury prevention warm-up programme dissemination and implementation (D&I) in youth team sports, including characteristics, contextual predictors and D&I strategy effectiveness.

Design Systematic review.

Data sources Seven databases were searched.

Eligibility The literature search followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Inclusion criteria: participation in a team sport, ≥70% youth participants (<19 years), D&I outcomes with/without NMT-related D&I strategies. The risk of bias was assessed using the Downs & Black checklist.

Results Of 8334 identified papers, 68 were included. Sport participants included boys, girls and coaches. Top sports were soccer, basketball and rugby. Study designs included randomised controlled trials (RCTs) (29.4%), cross-sectional (23.5%) and quasi-experimental studies (13.2%). The median Downs & Black score was 14/33. Injury prevention effectiveness (vs efficacy) was rarely (8.3%) prioritised across the RCTs evaluating NMT programmes. Two RCTs (2.9%) used Type 2/3 hybrid approaches to investigate D&I strategies. 19 studies (31.6%) used D&I frameworks/models. Top barriers were time restrictions, lack of buy-in/support and limited benefit awareness. Top facilitators were comprehensive workshops and resource accessibility. Common D&I strategies included Workshops with supplementary Resources (WR; n=24) and Workshops with Resources plus in-season Personnel support (WRP; n=14). WR (70%) and WRP (64%) were similar in potential D&I effect. WR and WRP had similar injury reduction (36–72%) with higher adherence showing greater effectiveness.

Conclusions Workshops including supplementary resources supported the success of NMT programme implementation, however, few studies examined effectiveness. High-quality D&I studies are needed to optimise the translation of NMT programmes into routine practice in youth sport.

Data availability statement

Data are available in a public, open access repository. Not Applicable.

https://doi.org/10.1136/bjsports-2023-106906

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WHAT IS ALREADY KNOWN ON THE TOPIC

Neuromuscular training (NMT) injury prevention warm-up programmes are effective at preventing injury rates in youth sports. However, for proper dissemination and implementation (D&I) by multiple stakeholders, barriers such as low adoption, adherence and lack of time must be addressed.

WHAT THIS STUDY ADDS

There are limited high-quality research studies to facilitate the widespread adoption of, and improved adherence to, NMT programmes. Few studies used D&I theories, frameworks or models. Programme flexibility is a common barrier to implementation; adaptation of NMT programmes to fit local contexts is imperative. Comprehensive workshops and supplementary resources currently support the success of NMT programme implementation.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

Promotion of NMT programmes as the standard of practice is essential to increase practical D&I of these programmes, and thus reduce the burden of youth sport injuries. This work provides some directions for stakeholders, including researchers, implementation support practitioners and youth sport policymakers, on current best practices for the delivery of NMT programmes in local youth sport settings. This work also provides the evidence base for more translational research efforts in youth sport injury prevention, a much-needed next step to optimise NMT programmes into youth sport practice.

Introduction

Youth (<19 years) sport participation provides numerous benefits, positively impacting physical and mental health. 1 Youth sport participation rates are high, with up to 90% of youth participating in sport globally. 2–5 However, with increased sport participation comes increased injury risk. One-in-three youth sustain a sport-related injury each year, leading to a significant public health burden with high healthcare costs. 3 6–8 Sport-related injuries may also result in long-term health consequences (eg, poor mental health, reduced physical activity, post-traumatic osteoarthritis). 7–9 Implementing injury prevention strategies is critical to mitigate the injury risk associated with youth sport participation.

Neuromuscular training (NMT) injury prevention warm-up programmes in youth team sport are effective in reducing injury rates by up to 60% and decreasing costs associated with injury based on randomised controlled trials (RCT) and systematic reviews. 10–21 NMT programmes include exercises that can be categorised across aerobic, balance, strength and agility components 22 23 and typically take 10–15 min. 24 25 Originally implemented with the intention of reducing non-contact lower extremity injury risk, 26–28 the effectiveness of NMT programmes has since been evaluated across numerous sports, age groups and levels of play and are associated with lower extremity and overall injury rates compared with standard of practice warm-ups. 12 20 21 25 In youth team sports, a protective effect has been demonstrated in soccer, handball, basketball, netball, rugby and floorball. 11 16 29–31 The International Olympic Committee Consensus Statement on Youth Athletic Development recommends multifaceted NMT warm-up programmes in youth sport. 32

Despite being a primary injury prevention strategy across youth sports, NMT programme adoption remains low. 33–38 For evidence-informed interventions to be successful and have a practical impact, pragmatic approaches derived from dissemination and implementation (D&I) science are necessary across multiple socioecological levels including organisation, coach and player. 36 Dissemination is defined as ‘the active process of spreading evidence-based interventions to a target population through determined channels and using planned strategies’. Implementation is ‘the active process of using strategies across multiple levels of change to translate evidence-based interventions into practice and prompt corresponding behaviour change in a target population’. 36

The aim of this systematic review was to evaluate current best practices for the D&I of NMT programmes in a youth team sport. The specific objectives of this systematic review were to: (1) describe the characteristics of identified D&I-related studies (studies with at least one D&I outcome directly or indirectly assessed as primary, secondary or tertiary outcome); (2) evaluate factors associated with the D&I of NMT warm-up programme across socioecological levels, including barriers and facilitators; (3) examine the effect of D&I strategies in delivering NMT warm-ups across multiple socioecological levels; and (4) examine the influence of D&I strategies on injury rates. Our protocol was registered in PROSPERO (CRD42021271734), and the review is reported in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines ( supplemental appendix S1 ).

Supplemental material

Search strategy and data sources.

A comprehensive search was developed with a librarian (KAH) in MEDLINE, incorporating four main concepts: child/youth, injury prevention, implementation/compliance/adherence and sports. The author team reviewed the final search strategy which was then piloted against the known key studies to ensure that the search was capturing relevant studies. Finally, the MEDLINE search was translated to the other databases. Searches were conducted 25 August 2021 (updated 16–18 August 2022; 5 September 2023). Search strategies are available in Supplemental Appendix S2 . Studies were identified by searching seven databases: MEDLINE(R) and EPUB Ahead of Print, In-Process & Other Non-Indexed Citations and Daily, Embase, Cochrane Central Register of Controlled Trials, Cochrane database of Systematic Reviews (all Ovid); CINAHL Plus with Full Text, SPORTDiscus with Full Text (EBSCO) and ProQuest Dissertations & Thesis Global.

Study selection and eligibility

All database search results were uploaded and duplicates were removed in Covidence (Veritas Health Innovation, Melbourne, Australia). Records were independently reviewed by authors in pairs (DL/IJS, CV/JK, KV/DL), starting with a screening of 50 randomly selected citations to assess inter-rater agreement with a threshold set at 90%. Each pair of reviewers performed title/abstract screening and full-text screening independently, providing reasons for exclusion at full-text stage ( figure 1 ). Any disagreements for exclusion, where a consensus could not be reached within pairs, were resolved by a senior author (OBAO). A secondary evaluation of included manuscripts was performed by senior authors (OBAO and CAE) to ensure appropriate inclusion. Study inclusion criteria were: (1) Participation in a team sport (male and female); (2) a minimum of 70% of participants as a youth (<19 years) or coaches of these youth teams; (3) reported dissemination and/or implementation outcomes (eg, self-efficacy, adherence, intention); (4) reported D&I strategies related to NMT warm-up programmes (ie, NMT delivery strategies, where applicable eg, in RCTs). Exclusion criteria were: (1) Studies evaluating rehabilitation programmes, non-team-based or physical education programmes; (2) non-peer-reviewed; (3) not English. The screening process was reported using the PRISMA flow diagram. 39

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Study identification Preferred Reporting Items for Systematic Reviews and Meta-Analyses flow diagram.

Risk of bias

To assess the risk of bias, three sets of paired reviewers independently used the Downs & Black (D&B) quality assessment tool. 40 The tool consists of a 27-item checklist (total score/33). A third senior reviewer (OBAO or CAE or AMR) resolved any disagreements. The rating of evidence and strength of recommendations were assessed using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) guidelines. 41–43

Frameworks/models

The proportion of studies that used D&I research theories/frameworks/models, including behaviour change frameworks/models, was examined to identify commonly used frameworks/models.

Efficacy-effectiveness orientation in RCTs

We assessed the components of 12 RCTs using the Rating Included Trials on the Efficacy-Effectiveness Spectrum (RITES) tool, as adapted by Maddox et al 44 RITES scores RCTs in systematic reviews based on a continuum of efficacy-effectiveness across four domains: Participant characteristics, trial setting, flexibility of intervention(s) and clinical relevance of experimental and comparison intervention(s) ( online supplemental table S1 ). 45 We modified the Likert grading system to classify studies depending on whether their emphasis was more on efficacy or effectiveness or balanced for both. Given that different aspects of each trial may fall in different places along the efficacy-effectiveness continuum, each RITES domain is scored independently and a composite score is not applicable. To minimise subjectivity, the RITES evaluation for included RCTs was completed by two reviewers (AMR and OBAO). Any disagreements were resolved through discussion to reach a consensus.

Study typologies and assessment of study relevance to D&I

The level of relevance of individual studies (RCTs and quasi-experiments) to D&I was determined based on the implementation-effectiveness hybrid taxonomy: Type 1 (primarily focused on clinical/intervention outcomes), Type 2 (balance focused on both clinical/intervention outcomes and D&I outcomes) and Type 3 (primarily or ‘fully’ (our adaptation) focused on D&I outcomes) studies. 36 46 For ease of interpretation of results, studies were rated considering three broad traditional research design categories (ie, hierarchy of evidence): RCTs, quasi-experimental and observational studies, including cohort, cross-sectional, pre-experimental, qualitative, mixed-methods and ecological studies. Observational studies were categorised as ‘fully focused’ observational-implementation (if only D&I outcomes were evaluated) or ‘partially focused’ observational-implementation (if a combination of clinical and D&I outcomes were evaluated’ D&I studies. 47 RCTs and quasi-experimental studies with Type 2 or Type 3 hybrid approaches were indicated as ‘highly relevant’ towards informing D&I best practices. Furthermore, observational-implementation studies that are fully focused on D&I were also indicated as ‘highly relevant’.

Data extraction

The extracted data included: study design, author, journal, year, population (eg, 13–17 years old female soccer players), participant demographics, D&I intervention strategies (eg, workshops, supplementary resources), D&I framework/model, control group strategies, D&I outcomes (eg, adoption, adherence, intention, fidelity, self-efficacy) and injury outcomes. Study design classification was completed based on data extracted and the process taken by authors, 48 which may have differed from the original classification. Furthermore, prospective, and retrospective cohort studies were consolidated into ‘cohort’ to improve ease of readability. D&I outcomes indicated as compliance were included in the appropriate adherence category as defined in Owoeye et al and described as ‘adherence-related’ outcomes, to maintain unified language across results; the full list is provided in online supplemental table S2 . 36 49–51 Based on the dose-response thresholds reported for NMT programmes within current literature, measures of adherence were used to indicate potential D&I effect ( online supplemental table S5 ). 24 36 52–54 Studies with cumulative utilisation (sessions completed/total possible) of ≥70%, utilisation frequency of ≥1.5 sessions/week or a significant association between D&I exposures and outcomes were defined as moderate-to-highly relevant and identified as having a potential D&I effect (ie, yes). Studies presenting cumulative utilisation <70%, utilisation frequency <1.5 sessions/week or no association between D&I exposures and outcomes were defined as low-to-no relevance (ie, no). Studies reporting both frequency utilisation and cumulative utilisation must both reach the established dose-response thresholds to be considered as having a potential D&I effect. D&I barriers and facilitators, factors influencing injury prevention implementation success and the identification of any frameworks used were also extracted and categorised into themes. Measures of potential effect for these results were summarised using OR, proportions and mean differences in D&I outcomes (eg, adoption, adherence). Injury-specific results were reported as incidence rate ratios, risk ratios, ORs or prevalence. D&I strategies were classified into various categories, including workshops, supplementary resources, personnel support, supervision and combinations of these strategies.

Equity, diversity and inclusion statement

Our author team is comprised of student and senior researchers across various disciplines with representation from low-to-middle-income countries. A variety of demographic, socioeconomic and cultural backgrounds were included in our study populations.

PRISMA flow, characteristics of included studies and risk of bias assessment

A total of 68 relevant studies were included from our initial and updated search yield of 9021 studies ( figure 1 ). Across included studies, 13 included only male youth participants, 13 included only female youth participants, 26 included both and 16 reported coach-focused findings. Sports represented were soccer (n=33), rugby (n=8), basketball (n=7), multisport (n=7), handball (n=5), floorball (n=3), field hockey (n=3), volleyball (n=1) and futsal (n=1).

Details of study characteristics and risk of bias are presented in online supplemental table S3 . D&B scores ranged from 4/33 to 24/33 (median=14/33) from a variety of study designs, including 20 RCTs, 16 cross-sectional, 9 quasi-experimental, 8 cohort, 6 qualitative, 3 ecological, 3 mixed-methods and 3 pre-experimental. The D&B scores for the two top D&I-related relevant studies—an RCT Type 2 study (n=1) was 21/33 and an RCT Type 3 hybrid study (n=1) was 17/33. Using the GRADE guidelines for the process of rating the quality of evidence available and interpreting the quality assessment, the strength of recommendations was ‘low’ given the multiplicity of designs. 42 43

Characteristics of current D&I-related studies

23 studies (33.8%) reported using a D&I /behaviour change framework/model. D&I frameworks included Reach, Effectiveness, Adoption, Implementation and Maintentance (RE-AIM) Framework (n=7), Consolidation Framework for Implementation Research (n=1), Precede-Proceed Model (n=1), Translating Research into Injury Prevention Practice (n=1) and Promoting Action on Research Implementation of Health Sciences (n=1) and the Adherence Optimisation Framework (n=1). Behaviour change models included the Health Action Process Approach (HAPA) (n=8), Theory of Planned Behaviour (n=1) and the Health Belief Model (n=1).

Assessment of study relevance to D&I

Two RCTs of 68 included D&I-related studies (2.9%) were identified as highly relevant to D&I best practices (ie, Type 2 or 3 hybrid approach). 55 56 18 (27.9%) RCTs reported a secondary analysis of D&I strategies 12 16 19 30 53 57–69 ; classified as Type 1 hybrids. Five (8.3%) quasi-experimental studies used Type 2 or Type 3 hybrid approach 22 70–73 ; the remaining studies (n=4; 5%) were classified as quasi-experimental Type 1 hybrids. 74–77 Many observational studies (n=17; 26.7%), 78–94 were highly relevant based on being fully-focused observational-implementation studies; 5 (6.7%) were partially-focused observational-implementation studies. 52 95–98 The remaining observational studies (n=17; 23.3%) were observational-implementation studies, 35 99–114 reporting D&I outcomes from a qualitative lens using interviews and surveys.

The RITES scores for the 14 D&I-related RCTs that examined injuries as primary outcome and D&I outcomes as secondary (Type 1 hybrid approach) are presented in table 1 . Almost all (13 of 14; 92.9%) of the RCTs focused mainly on intervention efficacy (as opposed to effectiveness) regarding the flexibility of NMT warm-up programmes. Cumulatively, effectiveness was rarely (7.1%) prioritised as a primary focus across all the 56 possible ratings of the RITES domains for all 14 studies. 50% of the domain ratings demonstrated efficacy as a priority and 42.9% of the ratings were indicated for a balance between efficacy and effectiveness.

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RITES domain scores for included trials

Contextual predictors of NMT warm-up programme implementation

50 (73.5% of total) studies identified ≥1 barrier or facilitator within their findings, with 10 (14.7% of total) specifically examining barriers/facilitators as their main objectives. Full list is provided in online supplemental table S4 . The most common barriers identified were time restrictions (n=30), 30 35 59 62 69 70 73 74 78 79 81 82 84 87–91 93 96 98 101 102 105 107 108 112 114 reduced buy-in/support (n=8) 62 75 84 87 105 110–112 and limited awareness of preventative effects of programmes (n=8). 74 84 103 104 107 109 113 Facilitators included comprehensive workshops from trained instructors (n=11), 53 71 78–80 84 90 96 99 100 112 accessibility of supplementary resources (n=10) 82 84 87 89 90 105 114 and uptake/support from multiple stakeholders (n=7). 56 67 84 101 103 105 112 Moreover, suggestions from multiple socioecological levels indicated that increasing programme education and support, increased sport-specific activities and improved awareness of preventive effects, influence NMT implementation success. 36 88 89 115 116 Figure 2 , adapted from Basow et al 117 illustrates the contextual factors reported in the literature. 117 This evidence-informed model shows the important barriers and facilitators that influence the end-user implementation of NMT warm-up programmes across the three key socioecological levels of change.

Contextual predictors of NMT implementation across multiple socioecological levels. (Adapted from Basow et al (2021)).(116). Notes. SE, self-efficacy. NMT, neuromuscular training. Bold represents top barrier(s)/facilitator(s).

51 (75%) studies used implementation strategies for NMT warm-up programmes. The most frequently used strategies were Workshops with supplementary Resources (WR; n=24), followed by Workshops with supplementary Resources, plus in-season Personnel support (WRP; n=14). Three studies employed both WR and WRP strategies. Other methods for implementation included only workshops (n=9), only supplementary resources (n=4), supplementary resources and personnel support (n=2), workshops with personnel support (n=1) and supervision (n=1). Note, some studies are duplicated throughout the table when multiple D&I strategies are compared. 22 53 56 86

The key D&I concepts that were reported within the included studies were adherence or adherence-related (eg, self-efficacy, translation and perception). Specific outcomes within these concepts were further examined from the individual study results. We did not have enough evidence to present a meta-analysis of the effect of D&I strategies on D&I outcomes. Therefore, online supplemental table S5 presents a qualitative summary of the relationships between reported D&I exposure and D&I outcomes. 40 studies reported adherence-related outcomes, of which 32 (80%) were indicated to have potential D&I effect. Studies using WRP (n=14) reported completing between 1.4 and 2.6 sessions/week and cumulative utilisation of 39–85.6%; 9 of these 14 studies have potential D&I effect. Studies using WR (n=24) presented utilisation frequency ranging from 0.8 to 3.2 sessions/week and cumulative utilisation of 55–98% of sessions; 16 of these 24 have potential D&I effect. In studies evaluating workshops only (n=9; 22%), frequency utilisation was reported between 1 and 2 sessions/week across eight of the nine studies and one study had 52% cumulative utilisation; two have potential D&I effect.

Effects of D&I strategies on injury outcomes

Three RCTs specifically examined the effects of the D&I strategies used to deliver NMT programmes on injury outcomes ( table 2 ). Two studies that compared both WR and WRP to supplementary resource only found no significant differences between strategies, 53 56 they reported reduced injury rates in the highest adherence groups by 56% and 72%, respectively. Another study comparing WR and WRP to a standard of practice warm-up found a 36% reduction of ankle and knee injuries when using WR and a 38% reduction in ankle and knee injuries without supervision. 22 There were no significant differences in injury rates between groups.

Injury Outcomes by D&I strategies and adherence

This study evaluated current literature to inform evidence-based best practices for the D&I of NMT programmes in youth team sport. To our knowledge, this is the first systematic review evaluating the D&I of NMT programmes in youth sport. To improve the practical implementation of NMT warm-ups, factors associated with implementation success and current best practices for delivering context-specific NMT programmes are required to be evaluated. 118 In this review, we found few D&I-related studies use D&I or behaviour change frameworks, theories or models to guide their research questions. We discovered the number of RCTs examining the effectiveness of D&I strategies for NMT programme delivery is limited. Common barriers to NMT implementation include programme flexibility and time restrictions; and the use of coach workshops and supplementary resources are currently the primary strategy in NMT programme D&I facilitation.

One-third of the included studies used a D&I framework or behaviour change model in their research work. The HAPA and RE-AIM models were the most frequently used. These models are a conceptual and organised combination of theories required to direct the design, evaluation and translation of evidence-based interventions (NMT programmes) and the context in which they are being implemented. 36 71 119 It is imperative for D&I studies to use these frameworks/models to fully understand specific implementation processes and contexts. Future D&I studies should consider using appropriate frameworks or models, including adaptations and combination of models to guide their specific aims.

Relevance to D&I

Across the relevant literature, a variety of designs and levels of evidence were included.

Of 68 studies, 7 (10.3%) were found to be ‘highly relevant’ toward informing D&I best practice (2 (2.9%) RCTs, 5 (7.4%) quasi-experimental). Other ‘relevant’ studies evaluated implementation as secondary objectives (Type 1 hybrid designs) and/or were of lower level of evidence. 33 observational studies were ‘highly relevant’ to D&I, assessing D&I outcomes and barriers and facilitators from a qualitative lens. While these studies are important for understanding D&I context, more high-quality and highly relevant studies such as RCTs and quasi-experimental designs using the Type 3 hybrid approach, or non-hybrid approach focused on solely evaluating the effectiveness of D&I strategies, are needed to advance the widespread adoption and continued use of NMT programmes in youth team sport.

Effectiveness versus efficacy

Effectiveness is indicative of an evidence-informed intervention’s readiness for practical implementation. 36 Findings from our RITES scores evaluation indicate that the majority of the RCTs had a primary focus on efficacy and not effectiveness. Although many RCT studies had a fair balance between efficacy and effectiveness for participant characteristics, trial settings and clinical relevance domains (≥50% of RCTs), there is a lack of flexibility in the development and evaluation of the evidence supporting current NMT warm-up programmes. These disparities regarding practical implementation have implications for D&I research and practice in this field. Current NMT programmes may need to be modified or adapted to the local context and evaluated further to improve implementation in youth sport settings.

Contextual considerations

In our Adapted Socioecological Model ( figure 2 ), we demonstrate that the utilisation of NMT programmes by individual players within youth team sport can depend on their coach adopting and implementing the warm-up, which may also be dependent on larger organisational systems. Barriers related to end-users’ success in wide-spread adoption and long-term maintenance can be moderated; however, researchers and implementers have to be intentional about tackling these recognised barriers and associated challenges 25 87 104 115 ; integrating the facilitators of successful implementation intending to reduce and address these obstacles is essential. The barriers and facilitators identified in this systematic review provide insight into the combination of D&I strategies that should be formulated and tested by D&I researchers in the sports injury prevention field.

Within the current review, lack of time, whether it be learning, instructing and/or practicing the programme, is a common barrier that plays a significant role in implementation. A recent narrative review focused solely on the barriers and facilitators associated with exercise-based warm-up programmes showed similar conclusions regarding time restrictions. 115 Collective themes within this literature for players, coaches and organisations found that reduced buy-in and support at different levels impacted the adoption of NMT warm-up programmes. The lack of awareness and knowledge of the injury prevention benefits of NMT warm-up programmes also presented major barriers to buy-in, leading to reduced implementation success. Future interventions should ensure that education about evidence-informed injury prevention outcomes associated with programme adherence is integrated within their D&I strategies.

D&I science is a growing field of study. A variety of D&I outcomes were identified such as self-efficacy, intention, reach, outcome expectancy and most commonly, adherence or adherence-related outcomes. These outcomes were evaluated using different D&I strategies for NMT warm-up programmes. The most commonly reported strategies were Workshops with supplementary Resources with/without in-season Personnel support. Evaluation of D&I outcomes showed that adherence or adherence-related outcomes were most frequently reported across studies. Various measures of adherence as defined by Owoeye et al (2020) were identified, including cumulative utilisation, utilisation frequency, utilisation fidelity, duration fidelity and exercise fidelity. 36

Adherence remains the most common D&I outcome in the sport injury prevention literature. 36 120 In this review, we defined adherence and adherence-related thresholds for a moderate-to-high dose-response to be ≥70% cumulative utilisation and/or ≥1.5 session/week to achieve the desired protective effects. This was done with consideration of pragmatism and a practical balance between programme efficacy and effectiveness given the existing literature. 24 91 32 of 40 studies (80%) from those with adherence or adherence-related outcomes had a potential D&I effect based on a moderate-to-high adherence or adherence-related outcome level. The use of WR and WRP was the most common D&I strategies for delivering NMT warm-up programmes. While there are several areas for improvement for the practical D&I of NMT warm-up programmes in youth sport settings, the use of comprehensive workshops and supplementary resources at various levels, particularly with coaches, appears to be the optimal best practice. However, only two ‘highly relevant’ D&I studies (RCTs) from the current systematic review presented conclusions based on the effectiveness of D&I strategies and outcomes specifically.

Many studies (n=26/68; 38.2%) included both male and female participants; however, no sex-differences were described. When examining D&I outcomes, only 7/26 (26.9%) had moderate-to-high adherence when looking at both male and female youth players. In total, 84.6% of the female-only (11/13) and 72.7% of the male-only studies (8/11) reported moderate-to-high adherence levels. These findings suggest greater attention and efforts for adherence and implementation of NMT programmes in the male youth team sport setting may need further consideration compared with the female youth sport context.

Of the preliminary evidence for Type 2 and 3 hybrid designs, the literature highlighted in the synthesis of this data that WR are effective strategies in injury prevention and showed more moderate-to-high adherence levels. Given that most studies are doing some form of WR, adding in-season personnel support does not increase the protective effect and may be less sustainable given that resources, time and support are significant barriers to the D&I of these programmes.

Additionally, greater implementation and programme buy-in were found in studies where uptake of these NMT programmes was supported across multiple stakeholders, particularly at the organisation level. 19 67 90 103 112 Catering to programme deliverers (coaches, organisations, parents) and evaluating their awareness, perception and self-efficacy may help further inform our understanding of D&I and how we can best work to promote programme uptake further.

D&I strategies and injury outcomes

The findings from this systematic review suggest that while various D&I intervention strategies are effective at reducing injuries in youth team sports, the ranges of injury rate ratios are similar across studies employing different strategies (32–88% lower injury rates across WR strategy studies and 41–77% lower injury rates across WRP strategy studies). 22 53 56 Although this was not the proposed evaluation of these studies, our findings demonstrate that the use of workshops may influence D&I success and the availability of supplementary resources alone may not be efficacious. Future evaluation of the influence of delivery strategies should be considered.

Future directions

Using facilitators to reduce barrier burden.

Regarding NMT strategy evaluation, our findings show that most of the current programmes focus on efficacy over effectiveness, particularly in the aspect of intervention flexibility; this suggests a need for the adaptation of NMT programmes to fit local contexts. NMT programme developers should consider more enjoyable and user-friendly exercises that include sport-specific activities (eg, ball work, partner drills, tags). Increasing variations also improves player buy-in and increases intrinsic motivation. At a coaching level, workshops on NMT programmes should include evidence-informed education on the injury prevention benefits and should incorporate content addressing coach self-efficacy to enhance implementation quality. 16 100 121 An ongoing pragmatic evaluation of NMT programme effectiveness is warranted as they undergo adaptation to local contexts.

Organisations have expressed limited knowledge and education for implementation as a significant barrier to successful NMT programme use. 90 99 101 105 112 115 122 Implementers should look to provide accessible resources and encourage further support from multiple stakeholders, including the governing bodies. This could lead to policy changes within the club and result in greater uptake of these programmes long-term. Collaborations among stakeholders (researchers, youth sport administrators, coaches and players) in programme development, evaluation, D&I are necessary to improve efforts for impactful practical translation of programmes.

Research recommendations

The support for NMT programmes within youth sport is extensive. 28 Although these programmes have been shown to be effective for injury prevention in many sports, 10 11 sport representation across D&I studies in our review was limited. Scaling up of NMT programmes and supporting continued research into other sports is vital for increased context-specific D&I of these programmes to reduce the overall burden of youth sport injuries.

Compliance and adherence were often used interchangeably, despite having distinct definitions. Although their mathematical calculations are similar, these two constructs are contextually different. Compliance refers to individuals conforming to prescribed recommendations in controlled intervention settings, 123 while adherence refers to the agreement of an individual’s behaviour to recommended evidence-based interventions in uncontrolled settings. 36 Standardised definitions should be considered more frequently by researchers to build on current knowledge and inform future D&I research.

Using D&I frameworks/models can improve NMT programme implementation success in a practical setting. 71 124 Application of D&I frameworks/models, including behaviour change models, 124 is limited in injury prevention and this is reflected in the current systematic review. Future studies should use D&I frameworks/models to help guide the implementation of these NMT programmes. In doing so, researchers can gain a better understanding of the contextual and behaviour change aspects related to youth sport injury prevention. 115

Limitations

Given the broad nature of our research question, specific results were required for inclusion. Despite being specific to our objectives, our limitations set for participant age range, team sport settings and English language studies only, may have resulted in missing other studies that evaluated D&I interventions and outcomes related to NMT programmes.

Due to the heterogeneous nature of studies, meta-analysis was not possible for any of our objectives. Inclusion of various study designs, although comprehensive, impeded this process and resulted in inconsistent injury and adherence definitions across our population of interest. Furthermore, the subjective nature of many qualitative studies included may have resulted in variability within the data extracted. With the varied definitions used for each specific outcome and design, we looked to consolidate the terminology used into more succinct and unified language and we encourage this to be employed by researchers.

Methodological flaws existed in the included studies that may warrant caution about the interpretation of our conclusions. For example, many of the included studies did not include power calculations or reported low power, increasing the chance of Type 2 error. Further, many studies did not consider confounding or effect modification in their analyses or failed to report the validity of measurement tools used for injury data collection. We also acknowledge that publication bias may have favoured the inclusion of studies demonstrating significant findings (eg, effectiveness, efficacy). By considering quality assessment as an objective, we aimed to account for these limitations.

There was limited evidence supporting the effect of D&I strategies on D&I-specific outcomes. There were only two high-level evidence (RCTs) studies in this review that directly discussed the matter of D&I strategies on D&I outcomes. 55 56 D&I-related outcomes were evaluated as secondary objectives in other high-level evidence studies, therefore, we could only examine the relationship between D&I strategy and outcome to assess if the strategy used resulted in moderate-to-high adherence levels, given our pre-established thresholds.

Conclusions

This systematic review demonstrates that: (1) Few D&I-related studies are based on D&I or behaviour change theories, frameworks or models; (2) few RCTs have examined the effectiveness of D&I strategies for delivering NMT programmes; (3) programme flexibility and time restrictions are the most common barriers to implementation and; (4) a combination of coach workshops and supplementary resources are currently the primary strategy facilitating NMT programme D&I; however their effectiveness is only evaluated in a few studies. This systematic review provides foundational evidence to facilitate evidence-informed knowledge translation practices in youth sport injury prevention. Transitioning to more high-quality D&I research RCTs and quasi-experimental designs that leverage current knowledge of barriers and facilitators, incorporates Type 2 or Type 3 hybrid approaches and uses behaviour change frameworks are important next steps to optimise the translation of NMT programmes into routine practice in youth team sport settings.

Ethics statements

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Ethics approval

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Supplementary materials

Supplementary data.

This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.

• Data supplement 1
• Data supplement 2

X @carlavdb_, @amraisanen, @KatiPasanen, @Kat_Schneider7, @CarolynAEmery, @owoeye_oba

Contributors DL, CE and OBAO contributed to development of study proposal and design. DL, CvdB, AMR, IJS, KV, JK, AH, CE and OBAO conducted search, study selection and screening, data extraction and synthesis and quality assessment. DL led the writing of the manuscript and was the guarantor for the project. All authors contributed to drafting and revising the final manuscript. All authors approved the submitted version of the manuscript.

Funding This study was funded by Canadian Institutes for Health Research Foundation Grant Program (PI CAE).

Competing interests OBAO is a Deputy Editor for the British Journal of Sports Medicine. CE, KJS and KP are Associate Editors for the British Journal of Sports Medicine.

Provenance and peer review Not commissioned; externally peer reviewed.

Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.

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