High-risk female athletes may demonstrate a preferential, increased coronal plane load strategy, as opposed to a sagittal plane load absorption strategy, during dynamic sport related activities, which may destabilize the knee and load the ACL [4
]. This strategy may be an ineffective technique for force dissipation during landing and cutting tasks. Moreover, it may also increase the risk for ACL injury [6
]. When neuromuscular training programs are implemented, female athletes achieve improvements in athletic performance, movement biomechanics and reduced ACL injury risk [9
]. Though these data may have been collected before many programs were widely instituted, the known positive effects measured in laboratories have not led to decreased national injury rates [15
]. The lack of overall reduction of injuries suggests that further work is needed to determine the most efficient training methods. If determined, these program may be more effective to influence coaches and athletes to institute effective programs and ultimately reduce ACL injury rates. The current study results, which define the potential to target high-risk females based on their knee abduction moments (>25.25 Nm), give evidence for the efficacy of a more "targeted" approach to the current neuromuscular training paradigms.
Cadaveric and modeling studies have shown that increased knee abduction loads directly elicit large ACL loads [30
]. In vivo data also indicates that females who demonstrate increased knee abduction during landing activities are at increased risk to sustain an ACL injury during competitive play [6
]. Increased neuromuscular control of dynamic lower extremity valgus, especially knee abduction, may be necessary to prevent loads that can cause ACL injury[6
]. Cumulatively, these data may indicate that interventions aimed specifically at reducing ACL injury risk in female athletes should focus to reduce mechanisms that underlie increased knee abduction loads [6
The current study design focused exclusively on evaluating the training effects on knee abduction load. This was done by utilizing abridged versions of a comprehensive training protocol shown to alter biomechanical factors related to increased ACL injury risk in female athletes [6
]. It was hypothesized that high-risk female athletes would decrease knee abduction moments after training, while low-risk and control athletes would not show changes at post-test measurement. The results of this study supported the hypothesis, indicating that females that demonstrate increased knee abduction loads (>25.25 Nm) may be able to reduce this torque through training, while low-risk athletes may not reap as much benefit from the training program. If this is the case, a logical extension suggests that it is more important to target athletes identified as "high-risk" for injury prevention training programs than those identified as "low-risk." Hewett et al. [19
] evaluated the available literature and developed a research design to test the effects of neuromuscular training on measures of lower extremity neuromuscular control. They determined that female athletes can demonstrate either a net knee abduction torque or knee adduction torque when landing from a jump. When evaluating only those females who displayed a predominately net knee abduction torque they determined that significantly reduced peak landing forces and knee abduction torques could be attained after six-weeks of neuromuscular training. While the sample size of females evaluated in that study who demonstrated an overall net knee abduction moment was relatively small (n = 4), the data provides initial evidence that relatively large (> 50%) reductions in coronal plane moments can be achieved with preseason neuromuscular training [19
Lephart and colleagues [12
] evaluated the effects of two different protocols comprised exclusively of plyometrics or basic resistance training (20–30 repetitions of bodyweight and band resistance exercises) on 27 high school female athletes to determine the potential biomechanical effects on female athletes. They found that neither training protocol significantly reduced the knee abduction torque. Myer et al. evaluated a comprehensive neuromuscular training protocol that was developed to reduce ACL injury risk and improve sports related performance measures [11
]. This comprehensive protocol successfully reduced knee abduction torques by 21%, but the potentially larger training effects may have been masked by low-risk females, as evidenced by the current study subjects classified as low-risk The low-risk subjects did not change their coronal plane mechanics with similar training regimens. If the subjects in these previous studies had been split into low-risk and high risk categories similar to the current study, larger reductions in knee abduction torque may have been observed, especially in athletes trained with plyometrics who reduced their maximum coronal plane knee torque by 36% (12.5 Nm but was not significant) [12
To further evaluate the effectiveness of neuromuscular training a systematic review of the published literature was previously performed, which yielded six published interventions targeted towards ACL injury prevention in female athletes[14
]. Four of six significantly reduced knee injury incidence and three of six significantly reduced ACL injury incidence in females. A meta analysis of these six studies demonstrated a significant effect of neuromuscular training programs on ACL injury incidence in female athletes (test for overall effect Z = 4.31, P < 0.0001) [14
Following the reports of positive biomechanical and epidemiological effects of interventions targeted to reduce ACL injuries in female athletes, the problem was evaluated with recent longitudinal data to determine if the incidence and severity of the problem had changed. Agel et al. [15
] performed a thirteen year (1989–2002) retrospective epidemiological study to determine the trends in ACL injury rates of National Collegiate Athletic Association soccer and basketball athletes. These authorsreported that male soccer players demonstrated a significant decrease in ACL injuries, whereas female soccer athletes showed no change over the same time period. Both female basketball and soccer players showed no change in the rates of non-contact ACL injuries over the study period and the magnitude of difference in rates (3.6 X) between their male counterparts remained unchanged[15
]. While these retrospective data analyses provide no evidence for decreased rates of ACL injuryover the study period, they may not directly reflect the effect or lack of effect of neuromuscular training programs on actual ACL injury incidence. The data collected and analyzed in the Agel et al. investigation ended in 2002[15
]. During this time, the positive benefits of neuromuscular training were just being published in medicaljournals and may not have been adopted into general practice by coaches and athletes. Future prospective, longitudinalinvestigations may show the positiveeffects of neuromuscular training on reduction ofknee injuries in female athletes.
In contrast to the findings of Agel et al.,[15
] Gilchrist et al. performed a large scale study to determine the incidence of ACL injury rate in a cohort of the same NCAA collegiate female athletes. This randomized controlled trial (RCT) studied interventional training as part of the practice and game warm-up. The authors demonstrated a significant reduction in non-contact ACL injuries in the study group during one competitive season of soccer [37
]. In support of the Agel et al. findings, Pfeiffer and colleagues found a 20 minute plyometric training regimen performed twice a week had no effect on ACL injury high school female athletes[38
]. In addition, Grandstrand and colleagues found that a commercially marketed warm-up program purported to reduce knee injury had no effect on high risk biomechanics [39
]. Cumulatively, there is evidence that demonstrates positive steps have been made toward prevention of ACL injuries in female athletes by using neuromuscular training [14
]. However, current ACL reduction program designs that offer non-targeted training protocols may not be the most effective.
A linear regression analysis was performed on the current data to determine the potential to change high risk knee abduction moments with training. The regression analysis suggested that a majority of the potential to reduce this risk factor was related to their initial measure. The analysis indicated that athletes with what is described in the current manuscript as "low risk" will have little, if any, reduction of knee abduction torque from neuromuscular training. However, athletes who demonstrate increased knee abduction moments, and were classified as high risk, appear to have a greater potential to change this outcome measurement. This inference has been previously demonstrated [40
], but is often ignored in clinical investigations [41
]. The current model indicates that neuromuscular training targeted to female athletes with high knee abduction loads will most likely provide the greatest potential to significantly reduce dangerous knee loading profiles that increase their risk for ACL injury. A recent investigation by Grindstaff and colleagues indicated that standard, non-targeted neuromuscular training programs require application to 89 female athletes to prevent one ACL injury[42
]. Theoretically, through identification of female athletes at greater risk for ACL injury, the numbers needed to treat to prevent an ACL injury could be substantially reduced. These investigations suggest that future interventional cohort investigations aimed to prevent ACL injury should apply their neuromuscular training intervention to high risk populations in order to improve the potential prophylactic effects. In addition, further research is warranted to delineate the most efficient training methods and the most effective means to increase widespread implementation of the most effective interventions.