Anterior cruciate ligament (ACL) injury is of major concern in the field of sports medicine and orthopedics. From 2002 to 2014 the rate of ACL reconstructions has increased by 22%. Those who experience ACL injury experience significant deficits in sports-related movement, including cutting, pivoting, decelerating, jumping, landing, and other functional movements. Concomitant injuries such as meniscal tear, chondral injuries, avulsion fractures, and collateral ligament injuries are common with ACL injury. ACL injuries primarily occur in young individuals who participate it cutting and pivoting sports such as soccer, basketball, football, and lacrosse. , Among adolescents and children, the rates of isolated ACL reconstruction and ACL reconstruction with a concomitant meniscal procedure increased noticeably from 2002 to 2014. Individuals aged 13–17 years experienced 37% increase in isolated ACL reconstruction, 107% increase in ACL reconstruction with meniscal repair, and 63% increase in ACL reconstruction with meniscectomy. The economic burden to society is significant, as one ACL injury (including long-term sequelae) currently costs approximately $38,000.
Although ACL reconstruction surgery has a high success rate, younger and more active patients have a higher likelihood of reinjury. Deficits in quadriceps strength, decreased postural control, and altered kinematics and abnormal joint loading contribute to the risk of reinjury. Additionally, the risk for a second ACL injury is significant (about 10%), and this risk doubles if the athlete returns to his/her competitive sport. In addition to reinjury, there is a risk of long-term complications that stem from ACL injury and reconstruction. Specifically, the risk for posttraumatic osteoarthritis, total knee replacement, and impaired knee quality of life 5–25 years after injury is high. In a systematic review, a prevalence rate of 48% for osteoarthritis 10 years following reconstruction surgery was reported.
It is well known that female athletes are four to six times more likely than their male counterparts to withstand a noncontact ACL injury. , Noncontact ACL injuries typically occur when the athlete is either landing from a jump or making a lateral pivot. Quadriceps dominance, leg dominance, and/or ligament dominance may contribute to the increased dynamic knee instability in females compared with males. The recovery time for ACL reconstruction is significant, approximately 1 year, and among the athletes who do eventually return to their competitive sport (i.e., 55%), their level of performance is likely to decrease.
These trends are worrisome and a focus on implementing ACL prevention programs among young athletes, especially females, is critical. For over two decades, investigators have developed a variety of ACL injury prevention programs as well as neuromuscular training (NMT) programs to address this problem. We know that injury-preventing NMT programs indeed reduce the risk of ACL injury by roughly 50% in female athletes ; however, there are many variations of NMT programs that differ greatly in their individual components. In this chapter, we aim to synthesize and present the most recent meta-analyses and systematic reviews to aid coaches, athletic trainers, team physicians, and parents understand and implement the most effective components of prevention programs for their young female athletes.
Neuromuscular Training Programs
Neuromuscular control is defined as an unconscious trained response of a muscle to stimuli regarding dynamic joint stability. It is a complex system of muscle activities including contraction, coordination, stabilization, postural control, and balance. In sports, neuromuscular control is crucial to perform jumping, landing, and pivoting tasks correctly and without injury. Neuromuscular control has been identified as an important factor when considering the differences in ACL injury risk and knee stability between males and females. Differences in muscle control, muscle activation, and movement patterns in male and female athletes have been implicated in the increased risk for ACL injury in female atheltes. , There have been many studies that describe these kinematic and biomechanical variations in sport movement patterns among males and females. For example, Chappell et al. investigated the “stop-jump” maneuver in particular and found that female athletes prepared for landing with decreased hip and knee flexion, increased quadriceps activation, and decreased hamstring activation, leading to increased ACL loading during landing when compared with males. Additionally, Ford et al. found that in middle- and high-school basketball players, females demonstrated greater knee valgus than males during a “jump-stop unanticipated cut” maneuver ( Fig. 5.1 ).
It has been shown that neuromuscular risk factors are modifiable through NMT, which can lead to decreased risk for injury and increased sport performance. In a study by Myer et al., NMT protocols that utilized both plyometric and dynamic balance tasks were studied in high-school female athletes. The investigators measured power, balance, strength, and landing force before and after training. The female athletes were all able to decrease their standard deviation of center of pressure during hop landing tests, as well as increased hamstring strength and vertical jump measurements, after a three times per week NMT program for 7 weeks.
The kinematic demands of sports differ and thus may be an important consideration when implementing NMT programs. As the majority of female ACL injuries stem from noncontact movements (i.e., cutting and pivoting maneuvers or jump landings), the frequency of these movements in their respective sports should be taken into account. For example, in a study by Cowley et al. the researchers assessed differences in cutting and landing tasks among high-school female basketball and soccer players and concluded that sport-specific NMT may be warranted with soccer players focusing on training for pivot and cutting movements and basketball players on jumping and landing mechanics. In a 2014 systematic review by Michaelidis et al. ACL injury prevention programs were assessed on their effectiveness in different sports. The authors concluded that training programs for soccer and handball athletes require sport-specific agility training, while jump-focused sports, such as basketball, should involve high-intensity plyometrics.
Historically, NMT programs have included a mix of strength training, plyometrics, balance exercises, and stretching to address muscle imbalances and develop control over muscle activation. Many studies have confirmed that NMT and injury prevention programs are a cost-effective strategy. In a 2018 study by Lewis et al., ACL injury prevention programs prevented 3764 lifetime ACL ruptures per 100,000 individuals (i.e., a 40% reduction in ACL injuries). Subsequent cases of osteoarthritis and total knee replacement procedures are also averted through the utilization of ACL prevention programs. Marshall et al. also investigated the economic impact of NMT programs compared to traditional warm-up strategies in youth soccer. A 38% reduction in injury risk and a 43% reduction in healthcare costs was found in the NMT group compared to the control. The authors projected that in 58,100 youth soccer players, an estimated $2.7 million in healthcare costs could be avoided over the duration of one season of implementation of an NMT program. Although NMT programs have been determined cost-effective, there is still considerable variation among programs, warranting an analysis of the most common and effective components of these prevention strategies. In the following section, common and effective components of ACL injury prevention programs will be reviewed.
Neuromuscular Training Program Characteristics
In 2018, the American Academy of Sports Physical Therapy published clinical practice guidelines for exercise-based knee and ACL injury prevention. They recommend a number of specific programs for reducing ACL injuries in athletes, including but not limited to HarmoKnee, Prevent Injury and Enhance Performance (PEP) program, and Sportsmetrics ( Table 5.1 ). These exercise-based prevention programs employ numerous intervention strategies, including proprioceptive training, NMT, strengthening exercises, stretching, agility, and plyometric exercises. Additionally, many programs employ instructors or coaches to give feedback to athletes on their performance of specific movements and exercises, particularly jump landing movements. ,
|PEP||RCT||NCAA division I female soccer players: control, n = 852; intervention, n = 583||12 Weeks through soccer season (15–20 min, 3 times per week)||Intervention group had lower ACL injury rate in practices ( P = 0.01), in late season ( P = 0.03), and in noncontact ACL injuries in those with a history of ACL injury ( P = 0.05)||Flexibility |
|HarmoKnee||Cohort||Female soccer players aged 13–17 years: intervention, n = 777; control, n = 729||4 Months (approx. 20–25 min, 2× per week, during preseason, and 1× per week in regular season)||Knee injury intervention: adjusted RR for compliance 0.17 (95% CI: 0.04, 0.64) |
Noncontact knee injury intervention: RR adjusted for compliance 0.06 (95% CI: 0.01, 0.46)
|Sportsmetrics||Cohort||High-school soccer, basketball, and volleyball players: female, intervention, n = 366; male, control, n = 434; female, control, n = 463||6 Weeks during preseason (60–90 min, 3 times per week)||Intervention females had lower rate of severe knee injury than control females ( P = 0.05) |
Intervention females had lower rate of noncontact knee injuries than control females ( P = 0.01) and control males ( P = 0.01)
In the 2019 meta-analysis and systematic review on ACL prevention programs for female athletes, the authors utilized robust quantitative and statistical methods to develop best-practice guidelines and elucidate the most effective components of various prevention programs. A total of 18 studies were included in the final analysis. Investigators determined that all NMT programs included some form of implementer training (i.e., brochure or workshop) to ensure proper implementation of the NMT program. Almost every program in the analysis included proper exercise instruction, exercise progression, and trainer or coach feedback. Importantly, programs that taught proper movement technique and knee stability during landing and other dynamic maneuvers during each training session were most effective. Surprisingly, some exercise components were found to be irrelevant (i.e., these programs were no more effective than those that did not include these exercise components), including balance, core strengthening, agility, and stretching. Although these components may be helpful in preventing other types of injuries (i.e., ankle sprains and muscle strains), they may be an ineffective use of time if the primary goal is ACL injury prevention. Some of these findings are consistent with those from other studies in the literature; for example, the prospective randomized intervention study by Söderman et al. which found no reduction in ACL injury in performing balancing exercises alone. However, Sugimoto et al. found a significant association in the reduction of ACL injury in programs that included core-strengthening/proximal control exercises. The authors discovered a recent trend for NMT programs to incorporate proximal control exercises, including plank, side plank, sit-ups/abdominal curl, push-ups, and upper body weight training. , There have been many laboratory-controlled studies that investigated the influence of proximal control on lower extremity muscle function and found associations among trunk, hip strength, and dynamic knee stability. , This finding is also consistent with the clinical practice guidelines that found level I evidence that female athletes, in particular, benefit from prevention programs that include trunk/core strengthening and stability exercises ( Fig. 5.2 ). Therefore incorporation of core strengthening and trunk stabilization appears to increase the effectiveness of ACL injury prevention programs.