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.
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:
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
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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