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Abstract
Background Injury prevention programme delivery on adherence and injury risk, specifically involving regular supervisions with coaches and players on programme execution on field, has not been examined.
Aim The objective of this cluster-randomised study was to evaluate different delivery methods of an effective injury prevention programme (FIFA 11+) on adherence and injury risk among female youth football teams.
Method During the 4-month 2011 football season, coaches and 13-year-old to 18-year-old players from 31 tier 1–3 level teams were introduced to the 11+ through either an unsupervised website (‘control’) or a coach-focused workshop with (‘comprehensive’) and without (‘regular’) additional supervisions by a physiotherapist. Team and player adherence to the 11+, playing exposure, history and injuries were recorded.
Results Teams in the comprehensive and regular intervention groups demonstrated adherence to the 11+ programme of 85.6% and 81.3% completion of total possible sessions, compared to 73.5% for teams in the control group. These differences were not statistically significant, after adjustment for cluster by team, age, level and injury history. Compared to players with low adherence, players with high adherence to the 11+ had a 57% lower injury risk (IRR 0.43, 95% CI 0.19 to 1.00). However, adjusting for covariates, this between-group difference was not statistically significant (IRR=0.44, 95% CI 0.18 to 1.06).
Conclusion Following a coach workshop, coach-led delivery of the FIFA 11+ was equally successful with or without the additional field involvement of a physiotherapist. Proper education of coaches during an extensive preseason workshop was more effective in terms of team adherence than an unsupervised delivery of the 11+ programme to the team.
Trial registration ISRCTN67835569.
- Adolescents
- Children's injuries
- Intervention effectiveness
- Soccer
- Injury Prevention
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Background
There is consistent evidence, based on prospective evaluation studies, that multifaceted neuromuscular warm-up programmes can reduce the risk of injury in youth football players.1–6 Among Norwegian 14-year-old to 16-year-old female football players, Soligard et al3 demonstrated an overall 32% reduction in injury risk in female youth football players participating in the FIFA 11+ injury prevention programme. Soligard et al7 further showed a greater protective effect in players with high adherence to the 11+, estimating a risk reduction of all injuries by 35% for those players participating in at least 1.5 structured warm-up sessions/week.
Despite the protective effect of many football injury prevention warm-up programmes, previous investigations among youth football players have suffered from moderate or unknown adherence to the injury prevention programmes.1–6 These and similar injury prevention interventions will not have significant public health impact if they are not widely accepted and adopted by the target population, coaches, athletes and other stakeholders in the sports community.8–10
In previous projects in community-based youth football, injury prevention programmes were largely delivered by coaches who initially were educated by clinical research personnel. Mostly, coaches were introduced to the injury prevention programme individually or through an instructional workshop where they had received theoretical and practical training in the programme and instruction on how to teach it to the team.2–6 ,9 ,11 Some research teams have in addition used combinations of a digital versatile disc (DVD)/video and other educational material (brochures/booklets and posters) to inform and teach coaches and players about the exercise programmes.2 ,3 ,6 ,12 Emery and Meeuwisse4 and Steffen et al9 also implemented a review of their injury prevention programmes with players and coaches at regular intervals on the field by a physiotherapist or other study personnel to ensure proper movement quality during exercise completion and to provide motivation. However, there is a paucity of information in the literature regarding the coaches’ delivery of the programme to the team and the quality of the exercise execution by athletes on the field. This knowledge gap has also been recognised as an issue for sports injury research more generally.13
Based on the positive effects of the FIFA 11+ football injury prevention programme and as part of its implementation strategy, the Federation Internationale de Football Association (FIFA) has developed educational material and created a website where all 11+ resources can be downloaded free of charge (ie, videos explaining the exercises, pocket-sized exercise cards for the training field, posters detailing the exercises and their common performance errors) (http://f-marc.com/11plus/11plus/). The success of delivering an injury prevention programme implemented through an unsupervised approach (ie, website delivery only) has not been evaluated previously. Injury prevention of football-related injuries through extensive coach education was successfully implemented in a countrywide campaign in Switzerland,11 but there are no prospective intervention studies evaluating different coach education or team implementation strategies in maximising team and player adherence and ultimately reducing injury risk in youth football or in any other sport.13 Furthermore, the added effect of a programme delivery, specifically involving regular follow-ups with coaches and players on the field to ensure proper execution and progression of exercises, has not been examined previously.
A better understanding of the impact of an optimal delivery strategy as well as the relationship between adherence and injury risk would allow stakeholders to optimise current injury prevention programmes and their delivery to targeted teams.14 The aim of the present study was to investigate whether a comprehensive, player-focused delivery of the FIFA 11+ injury prevention programme with regular on-field follow-up has a more significant effect on maximising team adherence as compared to either a self-regulated delivery (website) or a self-regulated coach-delivered programme following a coach-training workshop. In addition, the effect of programme delivery on injury risk in youth football players is explored.
Methods
Design and subjects
A cluster-randomised controlled trial (RCT) was conducted during one outdoor season (May–August) in 2011. The study population consisted of consenting coaches and youth female football players (ages 13–18) coaching and playing for teams from the Calgary and Edmonton Minor Football Associations or the Edmonton Interdistrict Youth Football Association, Alberta, Canada. Teams were randomly approached by clubs for recruitment in the early season after teams were formed (April to May 2011) and followed through the regular playing season and play-offs (August 2011). After receiving agreement from the head coach and a team designate to record exposure data for the study, the following inclusion criteria were met: playing in the top three divisions of play (tier 1–3), belonging to the under 16 (U16) or under 18 years (U18) age group, providing written informed consent to participate. Player exclusion criteria were: being injured or having had a systemic disease (eg, cancer, arthritis or heart disease) or neurological disorder (ie, head injury), which prevented full participation in all organised football activities at the start of the 2011 outdoor season.
Included teams were randomised to three study groups. These included an unsupervised control group or one of two intervention groups with a regular, coach-focused or a comprehensive, player-focused delivery of the 11+ programme. Ethics approval was granted by the Conjoint Health Research Ethics Board at the University of Calgary, Canada. To avoid contamination, teams were randomised by the clubs to a study group by a random number generation conducted by the study personnel not involved directly in recruitment or intervention delivery.
Intervention
The FIFA 11+ is a 20 min warm-up programme with neuromuscular training consisting of 15 exercises developed to prevent lower extremity injuries in football players. Exercises taught are grouped into three parts including initial and final running exercises with a focus on cutting, jumping and landing techniques (parts 1 and 3) and strength, plyometrics, agility and field balance components (part 2). For each of the six conditioning exercises in part 2, the 11+ programme offers three levels of variation and progression.3
Coaches from teams randomised to the ‘control’ group were provided with details for online access to the 11+ programme website (ie, videos detailing the exercises and other educational material for free downloading) (http://f-marc.com/11plus/). They were given no additional information or support regarding the 11+ delivery. Team coaches randomised to the ‘regular, coach-focused intervention group’ were provided with one preseason 11+ coach workshop by the study personnel and with copies of 11+ material (DVD or poster), which were also available on the 11+ website. In addition to a preseason 11+ workshop for coaches and receiving copies of the 11+ material, teams in the ‘comprehensive, player-focused intervention group’ were assigned a study physiotherapist who assisted the coach in the instruction and supervision of the 11+ programme to the players at team sessions following the workshop. These physiotherapists also attended a separate workshop to learn the 11+ programme and discuss the correct execution of the exercises prior to their contact with the study team. The role of the physiotherapist was to attend the team's practice sessions weekly to facilitate correct technique and progression of the programme components. Coaches otherwise led the warm-up programme.
All participating coaches were asked to perform the 11+ injury prevention programme with their team as a warm-up at the beginning of all practice sessions and parts 1 and 3 before match play (2–3 times a week). All coaches in any of the three study groups were given contact information of the study personnel to clarify questions and provide support by telephone when needed.
Exposure, 11+ adherence and injury surveillance
The total player exposure to practice, game play and to the 11+ programme was completed from the time of delivery of the coach workshop (coach-focused and player-focused intervention groups) or delivery of information for access to the 11+ website materials following baseline performance testing (control group) until play-offs were completed. All the teams identified a team designate (ie, team trainer, coach or manager) to be responsible for the exposure of data collection for every practice and game (ie, player attendance, time spent in practices and matches and number of 11+ sessions and single 11+ exercises performed). Team adherence to the 11+, the main outcome, was defined by three outcomes: team adherence to the 11+ (proportion of all possible sessions where the 11+ was delivered), the number of team 11+ sessions/week and the mean number of team 11+ exercises/session. Completeness of data collection was reviewed regularly as submitted by the study personnel.
Injury surveillance included a baseline medical questionnaire (including player demographics and injury history in the past 6 weeks), baseline on-field performance assessments and the completion of a player and coach survey regarding safety knowledge, attitudes and behaviours (data not included here). Daily participation exposure sheets and injury report forms, previously validated in youth community football,15 were completed throughout the season by a team designate and followed up and collected by a study therapist (separate to the 11+ physiotherapists). Players sustaining a football injury were directed by the team designate to a study therapist (physiotherapist or athletic therapist assigned to the team for the study) for assessment within 1 week of the injury event. While an injury report form was initiated by the team designate for details surrounding the injury event (session type, location, type of injury, cause of injury, etc), the team's study therapist completed the details of the injury report form including specific location, diagnosis and severity of injury. The injury definition included any football-related injury requiring medical attention and/or removal from a session and/or resulting in time loss from subsequent sessions.15 For any injury resulting in expected time loss of more than 1 week or any suspected concussion, the injured player was referred to a study sport medicine physician. Players could alternatively choose to follow-up with a family physician. Injury severity was classified based on the consensus agreement of injury definitions as slight, minimal or mild (0–7 days absence from football), moderate (8–28 days) and severe (>28 days).16 All study therapists and physicians examining the injury were blinded to study group allocation. Prior to the initiation of the 11+ program, all teams were asked to participate in field performance testing.17 The test session included the single-leg eyes-closed balance on an Airex Balance Pad (seconds),18 the star excursion balance test (cm),19 ,20 the single-leg triple hop (cm),21 and the jump-over-a-bar test (total number of 2-leg jumps in 15 seconds).22
Validation of team adherence
In addition to the team designate report, study personnel made random unannounced visits to study teams to validate team adherence and observe the execution of the programme. The study personnel observed the beginning of the practice or game session from a location where the coach would be unaware of their observation.
Sample size and statistical analysis
The sample size was based on a recent report using a similar approach with a mean team adherence of 77%.3 Given an estimated inflation factor of 1.88 for randomisation by cluster,3 ,9 a sample size of 30 teams and 360 players (120 players in each group) was estimated to detect a 20% difference in team adherence between the teams in the regular, coach-focused intervention group and the control group (estimated team adherence of 40%). The estimation was based on 14 players/team and an estimated player dropout rate of 15% (two-sided test; α=0.05, β=0.20).
All statistical analyses were performed using STATA V.10.0 (StataCorp, College Station, Texas, USA). Baseline characteristics, including preseason field tests were compared among the three randomisation groups, and data are presented as means with corresponding 95% CIs or frequencies and proportions.
Team adherence to the 11+ was treated as a continuous outcome and analysed using multiple linear regression to estimate the mean group differences with 95% CIs and adjustment for clustering by team. Following Soligard et al,7 player adherence to the 11+ programme was categorised into high, medium, and low adherence tertiles according to the players’ number of single 11+ exercises completed across all team sessions, independent of the study group.
The team designates’ reports on teams’ performed 11+ exercises were validated based on the agreement between teams’ reported and externally observed 11+ exercises. Data are presented as proportions of the agreement.
Injury rates in each of the three randomisation and adherence groups were estimated with clustering by team and player hours as the denominator. A Poisson regression model was used to estimate crude and adjusted incidence rate ratios (IRRs) with corresponding 95% CIs for each intervention group compared to the control group. These IRRs were also estimated using a similar model to compare the risk of injury based on tertiles of adherence according to the number of 11+ exercises completed (low adherence as the reference group). Data are presented as mean values with corresponding 95% CIs or frequencies with proportions, and significance was determined by non-overlapping CIs. All analyses incorporated team-cluster effects and were adjusted for age group, level of play and injury history. The level of significance was chosen to be α=0.05, and all tests were two-tailed.
Results
A total of 385 football players from 29 teams (11 teams in the control group, 8 in the regular, coach-focused intervention group and 10 in the comprehensive, player-focused intervention group) were included in the final analysis (figure 1). By the end of the preseason period to the beginning of the season in May, a total of 7 workshops were delivered to 35 head and assistant coaches from the 20 intervention teams and to study physiotherapists following the 10 teams in the comprehensive, player-focused intervention group. The median time to preseason baseline testing was 5 days following the 11+ workshop, ranging from 11 days before to 28 days after the workshop. For the 10 teams in the comprehensive intervention group, the median number of supervised on-field sessions by physiotherapists was 6, ranging from 3 to 8 team supervisions.
Table 1 summarises the baseline characteristics of all players. There were significant between-group differences for age group, playing level and injury history, while the distributions of playing position, as well as pre-season performance measures, were similar among the three randomised groups.
Flow of clubs through the study.
The mean number of match hours for players in the control and comprehensive, player-focused groups was similar, whereas players in the regular, coach-focused intervention group had a lower match exposure. For practice exposure, players in the two intervention groups had more hours of practice during the season than players in the unsupervised control group (table 2).
Baseline characteristics for players in the control, in the regular, coach-focused, and in the comprehensive, player-focused intervention groups, presented as frequencies (%) and means (95% CI).
Individual player exposure hours, injuries, team and individual adherence to the intervention for players in the control and in the regular, coach-focused and comprehensive, player-focused intervention groups
Team adherence to the 11+
Teams in both intervention groups completed a higher absolute number of 11+ sessions than control teams. However, based on a varying intervention length of these teams in the study, the relative number of 11+ sessions, measured as the number of team 11+ sessions/week, was similar between the groups. An examination of the number of team 11+ exercises/session reveals that teams in the control group performed on average a significantly lower number of 11+ exercises per 11+ session compared to the teams in the regular, coach-focused and comprehensive, player-focused intervention groups (table 2).
Teams in the comprehensive, player-focused and regular, coach-focused intervention groups had a 12% and 8% higher team adherence, respectively to the 11+ warm-up programme than teams in the control group, although these differences were not statistically significant when adjusted for the age group, level of play and injury history (table 3). Teams with a comprehensive, player-focused delivery of the intervention conducted 3.5 (95% CI 0.6 to 6.4) and teams with a regular, coach-focused delivery of the intervention performed 4.7 (95% CI 1.9 to 7.5) more 11+ exercises/session than teams in the control group, corresponding to an average of 10.8 and 11.5 compared to 8.2 exercises per 11+ session. However, when adjusted for the age group, level of play (tier) and previous injury in the past 6 weeks, these between-group differences were not statistically significant (table 3).
Adherence among players randomised to the control and to the regular, coach-focused and comprehensive, player-focused intervention groups
Injury risk among players
As many as 67 (81.7%) of the total 82 injuries were lower extremity injuries. The IRRs did not differ by study group (table 4). Most of the injuries (n=69, 84.1%) were of mild severity, resulting in 0–7 days absence from play.
Injury risk (injury incidence per 1000 h with 95% CI) among players randomised to the control and to regular, coach-focused and comprehensive, player-focused intervention groups and among players stratified into the high, medium and low adherence groups
Player adherence to the 11+ varied, and the cut-off values for the three groups of adherence based on tertiles were the: low adherence (0 to 108 11+ exercises during the season), medium adherence (109 to 209 exercises) and high adherence group of players (210 to 435 exercises). The unadjusted overall injury rate for players categorised into the high adherence group was 57% lower than the injury rate of players in the low adherence group (IRR=0.43, 95% CI 0.19 to 1.00). However, adjusting for the cluster, age group, level of play and injury history, this between-group difference in injury risk was not statistically significant (IRR=0.44, 95% CI 0.18 to 1.06). No other dose-response relationship between high and low adherence to the 11+ and injury risk could be identified (table 4).
Validation of team designate report forms
During the season, the project team conducted a total of 30 observations (11 practices and 19 matches) on 20 of the 29 study teams. These observations represented 4% of all the 707 potential sessions. The teams with no observation (n=9) were three teams from each of the control and the two intervention groups. A team designate report was missing for two observed sessions.
For the remaining 28 sessions, 85% of 11+ exercises were correctly reported (range 60–100%). There were no significant between-group differences in agreement between the reported and the observed 11+ exercises. There was no clear pattern identified in 11+ exercises where the team reports and observations disagreed. The observation also identified modifications of 11+ exercises as well as additional exercises performed during warm-up.
Discussion
This cluster-RCT is among the first to report results from a comparison of intervention delivery methods, focusing specifically on delivery of the FIFA 11+ to Canadian female youth football teams. So far, the extent to which knowledge of effective injury prevention programmes has been translated to the sporting field has not been known.10 ,23 The main finding of this investigation was that the method of delivery of 11+ injury prevention programme aimed at coaches with coach education, including a practical workshop at the beginning of the season, was more effective based on team adherence to the intervention than a web-based delivery of the programme to the team. There was no additional benefit from the regular involvement of a physiotherapist on adherence or injury risk for the teams in the comprehensive, player-focused intervention group. Analysing the number of team 11+ exercises/session, teams in the coach-focused and player-focused intervention groups performed a higher number of 11+ exercises/session compared to the teams in the control group who had unsupervised online access to the 11+. In addition, there seems to be a trend of a dose–response relationship between higher player adherence to the intervention exercises and a lower risk of injury, when adjusted for covariates. With caution, this could be interpreted as an important message for coaches: the more exercises a player does, the more effective the injury programme becomes, regardless of the method of programme delivery.
Programme delivery and team adherence
There is solid evidence in the literature that injury risk can be effectively reduced in youth football by implementing a neuromuscular prevention programme in RCT settings, when performed on a regular basis.3–6 However, there is a paucity of information examining the quantity and quality of exercise performance on the field. Furthermore, it is difficult to evaluate a dose-response relationship related to the effectiveness of an intervention programme without data examining adherence to the programme.10 Despite a mean adherence of 74% for teams in the control group, we found that an unsupervised delivery of the 11+ to control teams was not sufficient to attain as high adherence to the intervention as the two supervised intervention groups did. While the web-based material used in the control group delivery may have been thorough in content, it seems that a practical 2.5 h coach workshop, as was offered for both the coach-focused and the player-focused intervention teams, including take-home programme resources, seems to be favourable in improving adherence to the 11+. In addition to creating awareness among coaches for regular warm-up through neuromuscular training exercise programmes and providing continuous player feedback alongside, involving parents and other stakeholders and policy makers, could also be useful to increase programme uptake at the community level.24
The present findings suggest that coaches trained to deliver a team-based prevention programme following a theoretical and practical workshop are effective in attaining adherence regardless of additional follow-up by a physiotherapist. Similar adherence proportions in the two intervention groups give additional support to the strength of the 11+ as being a programme that coaches can deliver regularly. Future research should be directed at identifying factors, such as coach and player barriers and motivation, that may moderate the success of coach workshops and the distribution of programme resources.10 ,23 ,25 ,26
Adherence and injury risk
The reported injury rates among players in the present study are consistent with a previous RCT using a comparable age group performing a neuromuscular training prevention programme in Canada.4 The present findings also support the work of Soligard et al,7 who in a recent report on the 11+ revealed that the risk of overall and acute injuries was reduced by more than a third among players with high adherence compared to players with medium adherence, measured by the number of 11+ sessions throughout an 8-month season. Interestingly, for the present investigation, the risk of injuries was 56% (all injuries) and 48% (lower extremity injuries) lower among players with high adherence compared to low adherence to the 11+ exercises, although this risk reduction was not statistically significant. Based on the promising results from several neuromuscular football intervention studies,1–6 it appears that the observed results related to injury risk reduction are most likely related to the multifaceted content of exercise programmes, proper coach education and player adherence itself.
Methodological implications
The strengths of this study include the comprehensive evaluation of the effectiveness of implementation of the 11+ across several delivery methods, depending on the adherence and prevention of injury in a real world setting.10 This is the first study to directly examine the association between intervention delivery methods, intervention adherence and injury outcomes. As highlighted by others in the field,10 ,27 the present study examines effectiveness rather than efficacy in evaluating the most effective delivery mechanisms to inform coaches and support them in maximising uptake of such intervention by their players. Another strength of the present study is its design. A cluster randomisation of clubs was chosen to avoid contamination between the control and the two intervention groups and a careful monitoring of adherence. Also, the validation of the team designate report on adherence with high levels of agreement (81–89% agreement in the three study arms), based on researcher observation, suggests that the team designate report of adherence is reasonable.
The major limitation in this study relates to the potential bias of the availability of the 11+ resources previously, if coaches sought them out. It is possible that participating coaches may have been exposed to the 11+ or similar neuromuscular injury prevention programmes before the study. This bias could explain a dilution in effect, leading to non-significant results.
Also, differences in mean total team participation weeks in the control group (7 weeks) compared to the two intervention groups (10 and 11 weeks, respectively) constitute a limitation of this study. This difference is related to the random allocation of teams to the study group by clubs and the lag time between the club agreement for participation in the study and the team agreement through the head coach. Accessing the website materials later in the season by the coach, as was the case for most of the control teams, was maybe less likely, as the website information was provided at the time of recruitment. Regardless, a mean of 74% team adherence among the control teams is high compared to other youth football studies that have reported adherence.3 ,7 ,9
Another limitation of this study relates to the logistical challenges introduced by the inclement weather that was experienced throughout the football season, especially in the early season. This situation led to cancellations of many practice sessions and to rescheduling of games and practices. Also, related to the weather, rescheduling or cancellations, communication from the coaches to the field physiotherapists in the comprehensive, player-focused intervention group was poor. As such, the opportunities to complete and maintain the 11+ injury prevention warm-up sessions, as intended, were decreased and the opportunity for 11+ physiotherapists to follow-up their allocated teams on the field diminished. As a result, the team 11+ physiotherapists attended a team session on average only once every 2 weeks (median 6 supervised sessions in total). These logistical challenges, however, speak of the feasibility of such follow-up by a physiotherapist and of performing structured warm-up exercises regularly in community-based teams. Also, the exploratory analysis on the relationship between player adherence and injury risk might be biased, as most players with low adherence in this analysis are from the control group where team adherence was lowest. Player group allocation might not only have had an effect on adherence but also on the quality of the exercises conducted. This can subsequently have affected player injury incidence as well and needs to be interpreted with caution.
Finally, we also need to keep in mind that simply participating in a research study may change the study person's behaviour. In our case, coaches might have positively changed their willingness to incorporate the 11+ programme into their practice routines. This could artificially increase adherence in the control group. If this bias existed, it would hide differences between the groups, meaning that there may be more dramatic differences between groups than we observed in this study.
Practical implications
Coach education should continue to play an important role in the way a new neuromuscular injury prevention exercise programme is delivered to the football teams and players. Experiences from Switzerland demonstrated how an injury prevention programme could be successfully implemented countrywide by the coaches.11 Knowledge of injuries, injury prevention, attitudes and beliefs on the importance of injury prevention training is likely to be variable among coaches and players in youth sports.4 ,7 ,9 ,23 ,25 It is speculative whether or not coaches who are educated to a prevention programme through a practical approach, including a coach workshop, will be more likely to deliver the prevention programme with a better understanding of correct technique execution. Given the paucity of resources for injury prevention training and medical follow-up in youth community football, it is of major importance to establish injury prevention as a mandatory part of the coach education certification at all levels. More evidence is also needed to identify potential barriers and motivators for coaches in the use of an injury prevention programme and to measure determinants of change of behaviour over time.23–25 ,28 This would facilitate a better understanding of coach willingness to change current practices and behaviour and adopt such an intervention, thus establishing effective implementation strategies outside a controlled research environment.10
Conclusion
Comprehensive education of coaches during an extensive workshop at the beginning of the season proved to be more effective in subsequent team adherence to the coach-led injury prevention programme compared to being exposed to a web-based unsupervised delivery of the programme content. There was no additional benefit on team adherence or reduced injury risk with the on-field involvement of a physiotherapist to supplement coach delivery of the programme to players.
What are the new findings?
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Following a coach workshop, coach-led delivery of an injury prevention programme was equally successful with or without the additional field involvement of a physiotherapist.
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Proper education of coaches during an extensive preseason workshop was more effective in terms of team adherence than an unsupervised delivery of the 11+ programme to the team.
How might this paper impact on clinical practice in the near future?
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Coach education should continue to play an important role in the way a new neuromuscular injury prevention exercise programme is delivered to the football teams and players.
Acknowledgments
We thank all the research assistants, study therapists and physicians, coaches and players who participated in this project.
References
Footnotes
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Contributors KS, WHM, MR, JK, MB, JD, CF, GM and CAE were responsible for the conception and design of the study. KS, MR and CE coordinated the study and managed all aspects, including data collection. KS initialised and JK conducted the analyses, which were planned and checked with the other coauthors. KS, MR and CE wrote the first draft of the manuscript. All authors had full access to all the data and contributed to the interpretation of the findings and critical revision of the manuscript. KS, WM and CE are the guarantors.
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Funding This study was funded by the FIFA Medical Assessment and Research Centre (F-MARC), the Sport Injury Prevention Research Centre at the University of Calgary, supported by the International Olympic Committee (IOC) Research Centre Award, the Alberta Children's Hospital Research Institute for Child and Maternal Health Professorship in Pediatric Rehabilitation, supported by the Alberta Children's Hospital Foundation, and Alberta Team Osteoarthritis, supported by the Alberta Innovates Health Solutions. CF was supported by an NHMRC Principal Research Fellowship (ID: 565 900). The Australian Centre for Research into Injury in Sport and its Prevention is one of the International Research Centres for Prevention of Injury and Protection of Athlete Health supported by the IOC.
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Competing interests This project has received a grant from F-MARC, who is presented with two coauthors (MB and JD). No other relationships or activities appear to have influenced the submitted work.
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Patient consent Obtained.
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Ethics approval Conjoint Health Research Ethics Board at the University of Calgary, Canada.
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Provenance and peer review Not commissioned; externally peer reviewed.
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▸ References to this paper are available online at http://bjsm.bmj.com