WHAT IS ALREADY KNOWN ON THIS TOPIC

The 11+ programme has been shown to reduce overall injury rates and recovery time in male collegiate soccer players, but the impact of the 11+ on hamstring strain injuries, recovery times and exercise compliance has not been addressed.

WHAT THIS STUDY ADDS

Utilisation of 11+ decreased the risk of hamstring injury (HSI), but did not improve recovery time. High or moderate compliance with the 11+ were more effective at reducing rates of HSI compared with low compliance.

HOW THIS MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

Knowing that 11+ utilisation decreases the risk of HSI in addition to overall soccer-related injuries, there may be an advantage to using the 11+ compared with to prevention programmes that target HSI alone. Teams should emphasise compliance with 11+ to ensure maximal benefit.

Introduction

Soccer (football) is the world’s most popular sport, with approximately 300 million registered players globally.1 2 In the USA, soccer continues to grow in popularity with continued professional expansion3 and over 930 000 athletes at the collegiate and high school levels.4 With this amount of widespread participation, soccer-related injuries are increasingly more common and numerous studies have investigated the role of injury mitigation to address the injury risk associated with participation in soccer.1 5–12

Hamstring injuries (HSI) are one of the most common injuries associated with soccer athletes accounting for 12%–16% of all reported injury and have been the subject of multiple injury prevention research efforts.13–22 They occur more frequently in competition compared with training, reoccur at very high rates and present with persistent symptomology that often results in significant time loss, ranging anywhere from 17 to 90 days.23 24 To address the onus that injury has on the sport of soccer, the Federation Internationale de Football Association (FIFA) and its Medical Assessment and Research Centre (F-MARC) developed injury prevention programmes such as the ‘FIFA 11+’, more recently referenced as the ‘11+’, in an effort to improve strength, trunk control, proprioception and reduce the incidence and severity of all injuries incurred as a result of soccer participation.1 25 26 In men’s collegiate soccer, the utilisation of the 11+ programme has been shown to significantly reduce overall injury rates, severity of injury, ACL injury and time lost due to injury.1 However, the impact of the 11+ injury prevention programme on hamstring strain injuries in male collegiate soccer players has not been specifically investigated. The impact of compliance and implementation of HSI prevention has been debated vigorously, with the current literature suggesting that Nordic hamstring interventions that are not used regularly or used with low or poor compliance are not as successful at mitigating HSI as those who implement with higher compliance and fidelity.27–31 In relationship to harder endpoints such as number of HSIs and recovery time from injury, data are however lacking. As introduction of high-volume strength training for the hamstrings induces more delayed onset muscle soreness, understanding how little weekly exposure to this type of stimuli might still be protective of hamstring strain injury is of high importance.27

The purpose of this study was to investigate the effect of the 11+ utilisation on HSI rate and subsequent return to participation. We hypothesised that use of the 11+ would decrease the overall rate of HSI and reduce the severity of injury, reported as missed time for injured players, as was noted in prior studies ostensibly due to improvements in eccentric strength and neuromuscular control.1 32 We also hypothesised that higher compliance with the 11+ programme would optimise the reduction in HSI risk.

MethodsStudy design and participation

As reported previously,1 11 22 a prospective, cluster randomised controlled trial was performed in National Collegiate Athletic Association (NCAA) division I and II men’s soccer teams over one competitive fall season to examine the effect of the 11+ on overall injury rate, impact of compliance on injury and severity of injury and its ability to mitigate ACL injury.1 11 22 The inclusion criteria included current eligible student athletes who were participating in an NCAA division I or II member institution and, to avoid participant contamination, who had not participated in an injury prevention programme in the past four competitive seasons. There were 396 eligible teams with 299 teams meeting inclusion criteria. Sixty-five teams agreed to participate in the study (figure 1). IRB approval was obtained through the Quorum review board (IRB # 26182/1, Seattle, WA, USA). Each player provided informed consent in addition to obtaining signed documentation from each institution indicating that the coach and training staff had a full understanding of study implementation and involvement. Thirty-one teams were simply randomised to the intervention group and 34 teams to the control group. Four teams were lost to follow-up in the intervention group and 27 were included in the analysis. The intervention group received an instructional 11+ DVD, injury prevention manual and explanatory placards describing the details of the 11+ intervention (figure 2). The 11+ programme is a 15–20 min on-the-field dynamic warm-up programme used prior to games and training instructed to be completed two to three times a week throughout the course of the entire season. It includes strength, agility, proprioceptive and plyometric exercises and was designed to mitigate risk to injury that are most common in soccer players.25 There are three progressions to the 11+ programme to allow for individual variability within teams (beginner, intermediate and advanced). The dosage count of eccentric hamstring (Nordic) exercises specifically is 3–5 repetitions in the beginner group, 7–10 repetitions in the intermediate group and 12–15 repetitions in the advanced group per session. The 11+ was implemented as a dynamic team warm-up and was instructed to be performed in totality. The control group completed a traditional warm-up of their own choosing that was not defined as a prescribed injury mitigation warm-up. We used the Consolidated Standards of Reporting Trials checklist when writing this report.33 The NCAA coaches and Certified Athletic Trainers that were involved in the study were also involved with determining the reporting and the dissemination plans of our research.

Figure 1

Description of NCAA team randomisation and study flow. NCAA, National Collegiate Athletic Association.

Figure 2

Fifa 11+ posters.

Data collection

Each institution collected data using a secure internet-based injury surveillance system (HealtheAthlete; Cerner Corporation, Overland Park, KS, USA). Every athletic exposure, injury incurred (including HSI), utilisation of the 11+ programme and compliance data were entered weekly by the team’s certified athletic trainer (ATC) and verified by the research medical staff. Basic demographic data, position, mechanism of injury and playing surface were also documented by the medical team. The research staff monitored the data entry for each institution and sent email reminders if teams had not entered data for 14 days over the course of the season. On the completion of the season, the totality of the data entry was confirmed by each certified athletic trainer and the accuracy and completeness with their individual institution’s internal data collection system.

Exposure, injury data and compliance

On consenting to participate in the research study, each team provided a full roster to be entered into the HealtheAthlete injury surveillance system. The surveillance system was an electronic medical records system that was a data-secured, Health Insurance Portability and Accountability Act (HIPAA)–compliant site that utilised the Verisign secure second factor logon feature. The injury and exposure data for each player were entered by the team’s ATC. All injuries were entered weekly by the ATC and were verified and crosshatched with his or her institutional injury surveillance system at the end of the competitive season. The ATC indicated on which days the 11+ programme was completed, and which athletes participated in the training. The NCAA calendar was entered for each respective club to delineate the full soccer schedule, which commenced in August and ended in December (contingent on the success of the institution in the NCAA playoff tournament). No unique identifiers that would reveal the identity of the team or the athlete were visible to any of the research staff to preserve confidentiality. The operational definition for an athletic exposure (AE) was participation in any team practice or game during preseason or in season. We decided to use AEs over hours of playing time secondary to the domestic NCAA substitution rules.34 The current NCAA soccer rules allow for unlimited substitutions with a few minor restrictions. Teams can freely substitute without limitations, but there is no re-entry for a player in the first half, one re-entry in the second half and no re-entry if there is a game proceeds into overtime. Therefore, most NCAA exposure data are recorded in the literature as an athletic exposure versus hours of exposure. An injury was defined as any physical complaint sustained by a player that resulted from a soccer match or training, irrespective of the need for medical attention or time loss from soccer. An injury resulting in a player receiving medical attention was referred to as a ‘‘medical attention’’ injury and an injury that resulted in a player being unable to take full part in future soccer training or match play as a time-loss injury.35

Statistical analysis

Statistical analyses were conducted using IBM SPSS Statistics Editor for MAC V. 26 (IBM Corporation, Armonk, NY, USA). Descriptive and inferential tests were used to compare the control group and intervention group including frequency counts, proportion tests, χ2 tests, ORs and relative risk ratios. Relative risk ratios were presented with 95% CI. P values<0.05 were considered significant.

Equity, diversity and inclusion statement

The author group includes a female first author and consists of junior, mid-career and senior researchers in different disciplines including sports science, biomechanics, physical therapy and orthopaedic surgery. The authors are from three different counties including Denmark, Switzerland and the USA. We acknowledge that our study population included only male collegiate soccer players from the USA, but those players came from different countries, ethnicities and socioeconomic status. Additionally, the injury prevention programme being studied has application to soccer players of different genders and competition level throughout the globe without the utilisation of additional equipment or resources.

ResultsCharacteristics of HSI

There were 1525 male collegiate soccer players included in the study and 950 total injuries reported. HSI were the fourth most common injury (n=71, 7.5% of total injuries). The average age of players with HSI was 21 years old (±1.6) and the average return to play (RTP) was 9.6 days (±11.1). HSI were equally distributed across division I players (46.5%) and division II players (53.5%), and between grass playing surfaces (54.9%) and artificial turf playing surfaces (45.1%). The most common player position with HSI was defender (35.3%), followed by midfielder (33.8%), forward (26.8%) and goalkeeper (4.2%). There were nine different types of HSI mechanisms recorded, with sprinting/overstretch (35.2%) and dynamic overload (26.7%) as the two most common. For more information, see table 1.

Table 1

Characteristics of players with HSI

Risk of HSI

The control group included 850 athletes (34 teams), while the intervention group had 675 athletes (27 teams) (table 2). There were significantly fewer HSI in the intervention group16 versus the control group (55) (p=0.003). Using 11+ was associated with a decreased risk of HSI by 63% compared with the control group (RR=0.37, 95% 0.21 to 0.63). The number of AE to reduce one HSI was 24. There was not a significant difference in time to RTP between the control (9.4±11.2 days) and intervention groups (10.2±11.3 days).

Table 2

Control vs intervention group with HSI

Compliance

Compliance was defined in tertiles according to utilisation of the 11+ per week and reported here with reference to Nordic Hamstring Exercise (NHE) repetitions per week, as well (table 3). Low compliance ranged between 1 and 19 doses/season (<1 dose per week, 8–15 NHE repetitions per week), moderate compliance (MC) ranged between 20 and 39 doses/season (1 to <2 doses per week, 16–29 NHE repetitions per week) and high compliance was defined as utilisation >40 doses per season (2 or more doses per week, 30+ repetitions per week). Among the intervention group, 200 players reported high, 350 moderate and 100 low compliance (table 3). There were no significant differences in mean RTP between these groups. When comparing high versus low compliance, there was a decreased risk of HSI by 78% (RR=0.22, 95% CI 0.06 to 0.87). Finally, moderate versus low compliance was associated with a decreased risk of HSI by 67% (RR=0.33, 95% CI 0.11 to 0.97). High compliance reduced the risk of HSI by 33% (RR=0.67, 95% CI 0.17 to 2.55), compared with MC, but the result was not significant (table 4).

Table 3

11+ compliance and HSI

Table 4

Relative risk of hamstring injuries between intervention vs control group and by compliance

Discussion

In this study, we aimed to investigate the impact of 11+ on HSI rate, recovery time and compliance rates. We hypothesised that the use of the 11+ would decrease the overall rate of HSI, decrease the severity of injury and that higher compliance with the 11+ programme would optimise the reduction in HSI risk. Overall, 11+ decreases the risk of HSI by 63%, but has no effect on recovery times. Importantly, high to MC with the 11+ programme decreased the risk of HSI compared with teams with low compliance.

Risk of HSI

The 11+ prevention programme was developed to reduce the incidence of common soccer-related injuries.1 11 The findings confirmed that utilisation of the 11+ decreased the risk of HSI in male collegiate soccer players. However, the 11+ did not reduce missed time from these injuries. Better compliance with the programme optimised the reduction in overall HSI risk.

The reduction in HSI risk was similar to a recently published meta-analysis,12 critical appraisal,19 two other large randomised clinical trials17 20 and two cohort studies15 16 that studied the effect of the NHE programme on the incidence of HSI in male amateur soccer players in the USA and Europe. A meta-analysis by van Dyk found that injury prevention programmes that include NHE had a HSI reduction of relative risk by 51%.12 A study performed by van der Horst et al found that NHE decreased the relative risk of HSI by 57%,20 while Petersen et al found that using NHE decreased the relative risk by 70%.17 A prospective cohort study of one collegiate men’s soccer team reported a reduction of HSI from 6.2/1000 AE to 0.55/1000 AE, and a subsequent decrease in time loss due to injury.15 A potential advantage of utilising the 11+ over NHE is that the latter was developed to target only HSI, while the 11+ was designed to prevent all soccer-related injuries overall. Additionally, the 11+ includes other beneficial exercises for hamstring and posterior chain recruitment and is not limited to NHE in isolation. Regardless of the programme that one may choose to impart the HSI mitigation benefit, the evidence suggests that implementing HSI prevention techniques are effective in male soccer athletes. Studies that utilise injury mitigation consistently throughout the course of the season, as opposed to only in preseason or a partial season, tend to have lower injury rates due to prolonged exposure to the neuromuscular training benefit, with potential injury mitigation benefit improving as the season continues.36 37 The timing of the implementation, before or after training, indicated that there may be an increased strength benefit of performing the programme prior to training.16 This is consistent with the 11+ intended delivery method as a dynamic warm-up.

Time missed due to injury

The 11+ has been shown to significantly reduce time lost due to injury overall,1 but there was no significant difference in days missed due to HSI between intervention and control groups. The lack of difference in time missed between treatment groups was also similar in trials performed by van der Horst et al 20 and Petersen et al.17 A possible explanation for this finding may be overall low incidence within the intervention group (IG) and high variability in recovery from HSI due to factors including HSI injury severity and high incidence of recurrent injury.38 The average time missed of all players in this study due to HSI was 9.6 days (±11.1), which was less than players with HSI in the Australian Football League (21.2 days), professional rugby union (17 days) and Major League Baseball (27 days).38 This may be due to the fact that the athletes in this study are younger (18–23 years of age) and may not have a history of HSI. In the absence of MRI of each injury and historical data on each athlete in the study, intrasport comparison of severity of HSI is inherently limited. The elevated SD of both the IG and the CG reflect the high variability associated with severity of HSI and time missed due to injury, which may be reflected in the grade of injury at the time of diagnosis.

Dosage and compliance

One prior study also demonstrated that high compliance (HC) to the 11+ programme resulted in decreased severity of injury and fewer injuries overall.22 With respect to HSI, HC also resulted in significantly fewer injuries compared with players with low compliance (LC). Furthermore, players with MC had significantly fewer injuries than those with LC, while there was no significant difference in injury risk between players with HC and MC. These results suggest that even with MC, the 11+ will significantly decrease the risk of HSI. This finding is noteworthy, and has been corroborated by previous research publications, as the impact of a very low dose of eccentric hamstring exercise (16–29 repetitions per week) results in a significant decrease in HSI.17 20 The 11+, if performed two times per week for the intermediate and advance groups and three times for the beginner group, would easily meet this clinical beneficial threshold, as the prescribed dosage of eccentric hamstring per session is 3–5 repetitions (beginner) 7–10 repetitions (intermediate group) and 12–15 repetitions (advanced). This is a critical educational component, as compliance and programme adherence by players and coaches continues to be problematic across sport,22 and acceptable levels of programme adherence continue to be elusive.32 39–41 As a result, it is important to emphasise that players can substantially mitigate their HSI risk by performing a relatively low dosage of NHE (16–29 repetitions) per week while performing the 11+ programme. It is also important to define optimal programme fidelity and the minimum amount of adherence that provides significant benefit in order to promote feasibility among teams. At the collegiate level, division I teams tend to have more resources and larger training staffs compared with division II teams.42 For these reasons, it would be understandable that division I teams may more easily optimise implementation of injury mitigation, enhancing compliance and thus lower HSI compared with division II teams. In the previous article studying 11+ compliance in this population, 90% of division I teams demonstrated HC or MC versus 72% of division II teams.22 However, there was no difference in HSI between division levels in this study, which may further illuminate the notion that a low dose of NHE exposure imparts a protective benefit to the hamstring muscle group.

Limitations

There were multiple limitations to this study. Given the data were only collected from male collegiate soccer players in the USA, there may be a limit to generalisability of the results to female players and males participating at the professional or recreational level. The substitution rules of collegiate soccer are significantly different (NCAA soccer allows for unlimited substitutions and allows for re-entry in the second half and extra time) from those of professional and international play which could be particularly relevant to HSI due to the differences in duration, frequency and intensity of running during match play compared with the aforementioned groups. The truncated nature of the collegiate season provides a challenge to imparting the full neuromuscular benefit of the 11+ because of the short duration (4 months) of the season compared with domestic professional and European leagues.1 Participants in this study were also not blinded, which could have led to informational bias as being part of an injury mitigation study. The participants were predominantly of a similar age which makes it difficult to assess for the impact of that variable on the rate of and recovery from HSI injuries compared with older athletes. The athletes were diagnosed by an ATC and medical doctor, but MRI imaging for every HSI was not ubiquitously obtained for every injury, leading to challenges with objectively validating the grade of HSI. RTP was reported by each respective ATC, and due to the individualised decision making of the teams, a predetermined RTP criteria decision-making model was not adopted by the entire NCAA organisation due to the size and scope of the geographical reach. The 11+ is a team-based intervention, so individual compliance and programme fidelity was not captured in the data analysis, which limits the ability to assess compliance, qualitative performance of the programme with respect to individual HSI rate and severity of injury.

Data availability statement

Data are available upon reasonable request.

Ethics statementsPatient consent for publication

Consent obtained directly from patient(s).

Ethics approval

This study involves human participants and was approved by Quorum Advarra Institutional Review Board IRB # 26182/1. Participants gave informed consent to participate in the study before taking part.

Acknowledgments

The authors would like to thank the NCAA Athletic Trainers that assisted in the data collection for this study. We could not have completed this important work without your dedication and attention to detail.