High school sports are a vehicle for many students to gain physical fitness and develop character, discipline, and teamwork, but these benefits must be balanced against possible injury and sequela. After head/face, and ankle injuries, the knee is the third most-injured body site in organized US high school sports, according to National High School Sports-Surveillance Studies.1 From 2014 to 2015, the anterior cruciate ligament (ACL) was the most commonly injured knee structure; approximately 35% of female and 19% of male knee injuries involved the ACL.1 Overall, data from these surveillance studies have shown that girls' soccer and boys' football have the highest rates of ACL injuries at 12.2 and 11.1, per 100 000 athlete exposures, respectively.2 In boys and girls high school sports, ACL injury rates per athlete exposure have been shown to be similar, but in gender-comparable sports, such as soccer and basketball, girls experience 2.5 to 6.2 times more injuries than boys.2
The incidence of ACL injuries is increasing faster among pediatric and adolescent populations, compared with adults.3 This rise has been attributed to increased participation in sports among American youth, intensive training at younger ages, improved injury awareness, and better imaging modalities.4 Short-term, ACL injuries routinely sideline the athlete for the remainder of the season with pain, loss of function, and reconstructive surgery followed by 6 to 9 months of rigorous rehabilitation.5 Most student athletes with an ACL tear undergo autogenous or allogeneic graft reconstruction. A 2011 meta-analysis showed that most (82%) patients are able to return to sport after reconstructive surgery,6 although preinjury performance levels are achieved in only 43% to 65.7% of cases.6–10 Psychological and academic consequences are also reported. Both collegiate and high school student athletes who experienced ACL reconstruction reported deficits in classroom grades compared with controls.11,12 Long-term complications of ACL injuries are more concerning. Fifty-percent of those diagnosed with an ACL or meniscus tear develop osteoarthritis (OA) in the affected knee 10 to 20 years later,13 and reconstructive surgery has not been shown to reduce the incidence and severity of OA.14 Although newer surgical approaches may yield more anatomic repairs, it remains to be seen whether these techniques will reduce patient experience of OA later in life.
Given the significant challenges that athletes face with an ACL tear, prevention is most desirable. Evidence suggests that neuromuscular training (NMT) is an effective means to accomplish the goal of reducing ACL injuries among athletes by reducing dynamic instability.15–24 The regimens that have demonstrated statistically significant prevention of ACL injuries versus controls4,16,17,22–24 include multiple strength and plyometric exercises, and body mechanic instruction and feedback. Understandably, program compliance and duration are important: higher rates of ACL injury have been reported among shorter training regimens and those with lower athlete participation, compared with regimens with longer duration and greater participation.20,23 Neuromuscular training has been shown to be most effective when implemented both throughout preseason and in-season sport participation.16,17,22
Adaptability of sensorimotor function may, in part, determine who benefits most from NMT.25 Regardless of physiological explanation, young athletes,26 especially females, may have the most to gain. A meta-analysis of 14 studies reported a 72% risk reduction of ACL tears in female athletes <18 years of age who performed NMT in conjunction with their sport (soccer, volleyball, basketball, handball, and floorball).22 Odds ratios from further analysis indicated that mid-teen females (14-18) had statistically greater knee injury risk reduction compared with late teens (18-20) and early adults (20+).22 However, pooled findings from 9 other prospective trials that examined similar prevention programs for athletes in these same sports (minus floorball) documented a gender subgroup analysis in which training resulted in lower risk reductions among females (52%) compared with males (85%).19 A limitation of these analyses is that the trials do not include high-contact sports. This is noteworthy because although at least 70% of ACL injuries occurring among US high school athletes are noncontact in origin,27,28 60% to 75% of US football ACL injuries directly result from player-to-player contact.29
Young females are at an increased risk of incurring an ACL injury.1,2,30–32 Anatomic,30,31 hormonal,33,34 genetic,35 and injury mechanism36,37 differences between genders are documented factors. Of these, currently only 1 factor, injury mechanism, is amenable to prevention with training. Clinical imaging and diagnostic studies suggest that young females are especially prone to valgus-loaded injury mechanisms.36–38 Females' dynamic stability is negatively influenced with increased knee abduction, decreased knee flexion, asymmetrical landing, and increased ground reaction force (GRF).38–40 Zazulak et al prospectively studied the relationship between core neuromuscular strength and ACL injury. Interestingly, their study showed that core strength and control deficits correlated with ACL injury among female athletes, but not males.39 Although males also incur valgus-loaded tears, their injury mechanism may be more sagittal plane driven in nature.38,41 Despite gender differences in injury mechanism and rate, both males and females have been found to benefit from NMT,19 and the Canadian Academy of Sport and Exercise Medicine recommends NMT for the prevention of ACL injury in both boys' and girls' youth soccer.42
These findings and recommendations raise important questions regarding (1) the extent NMT is available to high school athletes, and (2) whether current use of NMT in high school athletics is impacting the incidence of ACL injuries in boy and girl athlete populations. The study reported here investigated Minnesota State High School League (MSHSL) schools' experiences of incorporating NMT into select fall sports, and the relationship between NMT and the incidence of ACL injuries during fall 2014.
METHODS AND ETHICAL CONSIDERATIONS
The survey developed for this study was based on the literature summarized above. As shown in the Appendix, Supplemental Digital Content 1, (see Appendix, http://links.lww.com/JSM/A121), the survey investigated which preseason and in-season NMT training modalities were performed (eg, strength training, plyometrics, body mechanic education, and aerobics), whether a licensed athletic trainer was part of the team, and how many ACLs were known to have occurred during the season among the students playing each sport (boys' football, boys' and girls' soccer, and girls' volleyball).
After approval by the University of Minnesota IRB (IRB #1409E53601), the survey form was sent to all athletic directors (ADs) at schools listed in the MSHSL web site (www.mshsl.org). The January 2015 cover letter invited them to return the completed survey indicating their school's experience during the fall 2014 athletic season. Four-hundred eighty-nine ADs received letters, 50 of whom did not have any of the sports of interest in their schools during fall 2014, leaving 439 potential responses. Two-hundred thirty-five surveys were returned indicating student experiences in one or more of the sports, for an overall 53.5% (235/439) return rate, with 57 of 137 (43.7%) metro schools and 146 of 263 (55.5%) rural schools returning their surveys.
The ADs reported that the students competed in teams that were distributed in competitive sections as assigned by the MSHSL. These sections are configured to create competitive brackets composed of comparably resourced teams.43 Consequently, larger schools with hundreds of students out for football play against each other. Often, section designations also differentiate geographically, with metro school teams playing against each other, and rural school teams playing in other sections.
Rates of ACL injuries and 95% CIs were calculated per sport, and adjusted for within school, team, and section experience using generalized estimating equations (GEEs).44,45 Potential selection bias due to nonresponse was partially accounted for by inversely weighting observed responses with probabilities of response.46 The weighted GEE analyses controlled for these differences and allowed us to understand the relationships between training and injuries across section assignments.
As shown in Tables 1 and 2, 235 Minnesota high school ADs (53.5%) returned surveys documenting the fall 2014 high school sports experiences of 611 teams and 27 335 students in football, volleyball, and boys' and girls' soccer. Among these athletes, 12 799 (46.8%) played football, 7672 (28.1%) played volleyball, 3111 (11.4%) were in girls' soccer, and 3753 (13.7%) in boys' soccer. More than two-thirds of these high school athletes received a variety of preseason and in-season NMT to complement their specific sport's coaching, and approximately half trained with a licensed athletic trainer.
Overall, the ADs reported that 167 (0.6%) of the student athletes sustained an ACL tear in their sport. The largest proportions of these injuries occurred with girls' soccer (n = 29, 0.9% of girls' soccer athletes) and football (n = 96, 0.8% of football athletes).
The analysis investigated which preseason and/or in-season activities were associated with the incidence of ACL injuries. The GEEs revealed that for this sample, varying training activities had different numbers of ACL injuries for each sport, and also documented differences in injury experiences between boy and girl athletes (Tables 3 and 4).
For boys, in football, strength training was associated with significantly fewer ACL injuries (risk ratio (RR), 0.088; 95% CI, 0.013-0.608). The lower number of ACL injuries among boys' soccer teams was associated with 3 activities: having a licensed athletic trainer work with the team (RR, 0.683; CI, 0.535-0.873), and the use of plyometric exercises (RR, 0.574; CI, 0.369-0.893) even if plyometrics were only engaged as an in-season exercise (RR, 0.668; CI, 0.489-0.912) (Table 3).
This was not true for girl athletes. In girls' soccer, none of the training activities were statistically related to the number of reported ACLs. However, if the girls' soccer team was based in a rural setting, the number of ACL injuries was remarkably and significantly lower (RR, 0.358; CI, 0.215-0.596). It is also important to note that for volleyball teams, preseason NMT training was actually associated with a larger number of ACLs experienced by the student athletes: the numbers were increased whether any (RR, 1.314; CI, 0.822-2.099) or all (RR, 1.504; CI, 1.268-1.784) of the training modalities were reported (Table 4).
This study confirms that many Minnesota high school sports teams are incorporating NMT in their athletic programming. Nevertheless, significant disparities in ACL injury by gender, sport, and geography were documented. For boys who played soccer, significant reductions in ACL injuries were associated with plyometrics (P < 0.05); for those who played football, fewer ACL injuries were associated with preseason strength training (P < 0.05). In contrast, for girls who played soccer in 2014, no combination of NMT experiences was associated with a reduction in ACL injuries; and for girl volleyball players, use of any NMT component was actually shown to be associated with increased ACL injuries (P < 0.01).
The lack of benefit NMT afforded females in this study was unexpected because multiple meta-analyses have reported that NMT prevents ACL injuries among young female athletes when core components (body mechanic instruction and feedback, plyometrics, and strength training) are implemented.16,17,22 The current findings may reveal that the NMT was not adequately executed for the female athletes in this study population, although LaBelle's research demonstrated that NMT delivered to varying extents by coaches significantly reduced ACL injuries among high school girls in her study.18 Besides this study, there are 2 other studies that investigated NMT and ACL injury experienced by volleyball players—however, these 2 studies did not record any ACL injuries among the volleyball players.15,47 In the current sample, there were 29 volleyball ACL injuries reported, and the analysis showed that utilization of NMT in their training puts these athletes at increased risk for ACL injury.
This study also documents that despite similar NMT programming, significantly fewer ACL injuries occurred among girls who played for rural soccer teams, compared with metro teams (P < 0.001). Schedules from the MSHSL web site indicate that the number of games played by urban and rural teams are comparable,48 but practice duration and intensity are unknown. Perhaps the intensity of training available in metro areas, access to additional training camps and other sporting events, and additional resources, puts metro athletes at a greater risk of injury, consistent with the observations of Sugimoto et al.4 This will benefit from further investigation.
In this study, boys' soccer athletes demonstrated the greatest potential benefit associated with the use of NMT, reflected in a lower injury rate relative to the other sports in question and when compared with previous data.2 Boys' soccer teams in this report had lower rates of ACL associated with plyometric training (P < 0.05), and associated with a licensed athletic trainer (P < 0.005).
Consistent with other trials, our findings revealed that high school football accounted for the largest number of reported ACL injuries: 96 of 167 (57%). Similar to results in previous studies, football and girls' soccer had the highest rates of ACL injury—0.8% and 0.9% per athlete per season, respectively. These rates were approximately double those of girls' volleyball (0.4%) and 4 times that of boys' soccer (0.2%). Outside the especially low rate for boys' soccer, these findings are consistent with those of previous studies.2,30,32
To our knowledge, this is the first study to examine the impact of NMT on ACL injuries in high school football. This sport is unique because most ACL injuries that occur in football are the consequence of contact.29 In contrast, most sport-related ACL injuries are noncontact in nature.27,28 In this study, teams that participated in strength training reported a lower incidence of ACL injuries compared with those that did not (P < 0.05). Further investigation is necessary to explore these benefits.
Suggestions for Future Research and Recommendations
This study confirms that using NMT as part of high school sport season training is associated with fewer ACL injuries among boy athletes, but no similar association exists for girl athletes. In addition to investigating the relationships between strength training and reductions in contact-related ACL injuries, the current results suggest that high school athletic programs use (and evaluate) mandatory gender- and sport-specific NMT exercises and activities.
As multiple investigators recommend, female athletes need NMT that counteracts inherent ACL injury risk factors.4,5,23,24,30,42 The mechanism of noncontact ACL injury in females involves significant valgus loading and collapse of a multifactorial etiology. Decreased core neuromuscular strength, increased knee abduction, decreased knee flexion, asymmetrical landing, and increased GRF explain this mechanism.36–40 Girls' soccer and volleyball teams that adopt NMT programs with multiple exercises within the domains below are likely to incur fewer injuries:
Proximal/Core and Lower Body Strength Training
Squats and lunges work to counteract dynamic knee valgus instability.5 Hamstring strengthening is especially important to counteract the quadriceps-induced aspects of ACL injury and dynamic valgus.24,49
Eccentric and concentric jumping exercises increase muscle power and minimizes strength imbalance between legs.5
A licensed athletic trainer or coach is able to recognize dynamic knee valgus to teach proper body mechanics and correct improper technique in an ongoing basis.5
Meta-analysis indicates that balance training alone is not preventive,24 but incorporating multiple balance exercises with strength training and plyometrics seems to further reduce risks.24,50,51
Frequency and Duration
For necessary neuromuscular changes to occur, NMT workouts should be at least 20 minutes and no less than twice per week for at least 6 weeks.5,17,23
The findings of this study need to be interpreted with caution. This retrospective cohort study reports the experience of only 1 year's fall sports season in a little more than half the schools in Minnesota, although it does report the experience of the population of schools participating in the MSHSL. The GEE analysis adjusts for the unevenness in the data, although we truly cannot know the experience of those who did not respond to the survey. The study used a unique survey instrument to investigate team participation in preseason and in-season NMT activities for various durations. Survey questions were based on the reported success of NMT and broader use of training programming in high school settings, as outlined in other studies.4,5,17,22 However, the current survey did not investigate the exact type and frequency of training exercises, whether the exercises were used throughout the season, or athlete compliance, nor did it ask about whether the injuries occurred in matches or in practice sessions; this information could have added insight to the findings. Future prospective and controlled studies that consider specific exercises and participation rates will help identify opportunities to bridge the gender, sport, and geographic gaps documented in this study.
This is the first study to examine the relationship between NMT, other training programming, and ACL injuries across state-wide youth athletic populations by sport, gender, and rural/metro geography. These results document significantly different ACL experience among 4 high school team sports—2 boys' (soccer and football) and 2 girls' (soccer and volleyball) sports. In this fall 2014 cohort, NMT was associated with fewer ACL injuries for male athletes, but not female athletes. This study also revealed a significant disparity between urban and rural girls' soccer ACL injury rates, independent of training.
The authors thank James G. Boulger, PhD for his support to this project. They also thank all the athletic directors in Minnesota who responded to the survey providing the information for this report.
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