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Adolescent girls who participate in competitive or recreational sports are at higher risk for anterior cruciate ligament injuries than adolescent boys, but through neuromuscular training and knowledgeable coaching some of these injuries may be preventable.
Over the past 20 years, the number of young athletes presenting with anterior cruciate ligament (ACL) injuries has increased, primarily because of the growing number of children participating in competitive sports at an early age and exposure to more intense levels of training, along with increasing awareness and detection of such injuries.1-3 Female adolescent athletes have the greatest risk of ACL injuries, with rates 4 to 6 times as high as for their male counterparts in similar sports.4,5 Some of these injuries may be preventable through neuromuscular training (NMT) programs. This article reviews the epidemiology, mechanisms, and risk factors for ACL injuries; describes the evidence for the protective effect of NMT in female athletes; and provides pediatricians with resources for educating patients, families, and coaches.
Knee injuries, especially those involving the ACL, are a significant concern for adolescent athletes. The ACL is 1 of 4 major ligaments that stabilize the knee. Its primary role is to prevent knee instability by keeping the tibia from sliding forward in relation to the femur. It functions secondarily to restrict excessive knee extension, varus and valgus knee displacement, and tibial rotation.6 Additionally, the ACL protects the cartilaginous shock absorbers of the knee (the menisci) from damage that could occur while jumping, cutting (rapid deceleration associated with a quick change in direction), and pivoting in sports.
ACL injuries have both immediate and long-term consequences for young athletes. Short-term consequences include pain and disability during the treatment phase, which involves surgery and 6 to 9 months of intensive rehabilitation before return to sports is considered. Treatment costs related to surgery, therapy, and rehabilitation are substantial. Estimates from 1999 to 2000 averaged $17,000 per injury and are likely to have significantly increased since that time.5,7 Furthermore, treatment of an ACL injury can considerably affect an athlete’s academic performance. For example, 36% of athletes in 6th to 12th grades undergoing surgery during the school year were noted to fail an examination upon return to school, compared with 0% of those undergoing surgery during a holiday or summer break.8 The potential consequences of an ACL injury may be more pronounced for girls than for boys. A study of high school athletes found that compared with boys in similar sports, girls were more likely to have surgery and less likely to return to sports after an ACL injury.9
Perhaps more serious are the long-term consequences of an ACL injury. For many athletes, ACL injuries can limit future participation in physical activity, which has well-known benefits for adolescents including enhanced self-esteem and academic success, improved bone health, and lower rates of obesity, diabetes, depression, and teen pregnancy.10-15
It has also been well documented that regardless of whether the ACL is reconstructed, those with an ACL injury are at 10-fold higher risk of developing early-onset degenerative knee osteoarthritis compared with the noninjured population.16,17 Lohmander et al reviewed 127 individual studies of follow-up after ACL rupture and/or surgery, most of which included subjects who injured their ACLs during their teenaged years, and found that on average, 50% had knee osteoarthritis with associated pain and functional impairment at 10 to 20 years after injury.16 This means that adolescents with ACL injuries have a high risk of suffering from chronic pain and functional limitations from knee osteoarthritis by their twenties or thirties.
Approximately 80% of ACL tears occur without any contact with another player, while the athlete is landing from a jump, decelerating suddenly, or quickly changing direction. Through video analyses of dozens of ACL injuries, researchers have noted that at the time of injury, the body’s center of mass was usually behind and away from the base of support; the knee was most commonly in full extension or close to full extension; and the lower extremity was in “dynamic knee valgus,” a position characterized by hip internal rotation and adduction, tibial external rotation, and foot eversion (Figure 1).17-19 These findings corroborate those from biomechanical studies both in cadavers and in vivo showing that the highest strain on the ACL occurs during isolated quadriceps contraction with the knee relatively straight.
FIGURE 1 Female athlete landing from a jump with the right leg in dynamic knee valgus-hip internally rotated and adducted, tibia externally rotated, foot everted-and with poor control of the center of mass, with her weight unevenly distributed between the right and left legs and trunk and pelvis tilted to the left.
ACL injuries are rare in children aged younger than 12 years.20 Ligament sprains, in general, are less frequent in younger age groups, presumably because ligaments are stronger than bones and growth plates at this age, and, therefore, skeletally immature children are more likely to sustain a fracture than a ligament sprain. ACL injury rates begin to increase at ages 12 to 13 years for girls and at ages 14 to 15 years for boys.21 Girls are 4 to 6 times more likely to sustain an ACL injury compared with boys participating in similar sports (Figure 23).5 This gender difference in ACL injury rates for girls peaks during adolescence, then declines in early adulthood.4 The segment of the population that accounts for the highest number of ACL injuries is female athletes aged 15 to 19 years. Girls’ high school sports associated with the highest rates of ACL injuries are soccer, basketball, and gymnastics, which account for 11.7, 11.2, and 9.9 injuries per 100,000 athlete exposures (an athlete exposure is 1 athlete participating in 1 practice or competition).3
Extrinsic risk factors
A study of approximately 3,000 high school football players showed that those wearing shoes with longer, irregular cleats placed at the periphery of the sole may have an increased risk of ACL injury, presumably because of increased friction at the foot-to-turf interface.22 Studies of football players also found that dry weather increased the risk of ACL injuries on natural grass.23,24
Intrinsic risk factors
Intrinsic factors that increase the risk of ACL injury include increased weight and body mass index, ligamentous laxity, subtalar overpronation, previous ACL injury, and female sex.25-28 One study found that the incidence of ACL injury in athletes who had had ACL reconstruction was 15 times greater than that of control subjects.25 Athletes with generalized ligamentous laxity were 2.8 times more likely to injure their ACL.26 Flexible hamstring muscles, a larger quadriceps angle (Q angle), a steeper slope of the tibial plateau, and a narrow intercondylar notch where the ACL is housed have been proposed as risk factors for ACL injury; however, existing data regarding these factors have been either insufficient or inconclusive.28
Similar to other ligaments, the ACL has receptors for estrogen, testosterone, and relaxin, which suggests that sex hormones may affect the mechanical properties of the ACL and thus influence the risk of ACL injury. However, data from studies investigating the effect of sex hormones on ACL injury risk have thus far been inconclusive. The female ACL does appear to have half a millimeter more laxity during the midpoint of the menstrual cycle. However, ACL injuries have been shown to cluster near the start of menses, at the polar opposite time in the cycle.29
The primary mechanism by which sex hormones influence ACL injury risk is likely to be through indirect effects on neuromuscular growth and maturation during puberty, rather than through direct effects on the ligament. During the pubertal growth spurt, as height and weight increase, control of these new body dimensions and the changing center of mass becomes more difficult, particularly during athletic movements such as landing, cutting, and pivoting. During puberty, boys undergo a large testosterone surge, which mediates significant increases in muscle mass and strength and allows them to better control their new body dimensions and changing center of mass during athletic maneuvers. Girls experience only a small increase in testosterone levels during puberty, resulting in a much smaller increase in muscle mass and strength, which may be insufficient to control their new body dimensions during athletic maneuvers.
Current evidence suggests that the primary reason girls are at greater risk than boys for noncontact ACL injuries is that girls tend to have less neuromuscular control of knee motion during athletic maneuvers. In other words, girls tend to use their muscles differently than boys when landing from a jump or quickly changing direction. Biomechanical studies have identified 4 neuromuscular strategies that are more common in girls and that may lead to dynamic knee valgus (Figure 1), a position that places the ACL at a high risk of tearing.
(1) Girls tend to use their quadriceps muscles much more than their hamstrings. Kinetic and kinematic analyses have found that during a jump landing or quick change in direction, girls have reduced knee flexion, increased quadriceps activity, and decreased hamstring activity compared with boys. This "quadriceps dominant” strategy has been shown to increase both anterior tibial translation and strain on the ACL.30 Notably, ACL strain is significantly reduced when there is co-contraction of the hamstrings.4
(2) Girls tend to have 1 leg stronger than the other, whereas boys tend to have equal strength in both legs. Asymmetry in leg strength promotes asymmetric weight distribution between the feet upon landing, causing a shift of the body’s center of mass away from its base of support, a position associated with increased risk of ACL injury.31
(3) Girls tend to have less core strength and stability, which makes it more difficult for them to control their center of mass and prevent it from shifting away from the base of support.19
(4) Girls tend to rely on bones and ligaments to stop joint motion, rather than contracting their muscles to control joint position and absorb the landing forces.31
Fortunately, unlike anatomic risk factors, which are largely nonmodifiable, these neuromuscular risk factors can potentially be modified through training.
Prevention of ACL injuries in female athletes
Various neuromuscular training (NMT) programs designed to strengthen hamstring and core muscles, improve balance, and teach athletes how to recognize and avoid dynamic knee valgus have been studied.5,32-37 Most of these programs have been shown to reduce ACL and other lower-extremity injuries. Pooled results from prospective cohort studies and randomized, controlled trials have demonstrated a 72% reduction in ACL injury rates among adolescent female athletes.31 This body of scientific research provides significant evidence to advocate that NMT be routine in girls’ high school sports.
NMT programs are somewhat variable with respect to the number and types of exercises included and the frequency and duration of training. Some studies used only 1 or 2 types of exercises, such as plyometric exercises (repetitive jumping to build muscle strength and power) and/or balance exercises, whereas others applied a more comprehensive approach, incorporating plyometrics, strengthening, stretching, and balance training.4,19 Pooled analysis of these studies showed that the most effective programs combined 3 key components: (1) progressive strengthening for the core and lower extremities, (2) plyometrics, and (3) feedback-driven technique modification.38,39 NMT programs that included only balance training were not effective in reducing ACL injury risk. Additionally, compliance rates were highest with coach-led programs.38
Progressive strengthening exercises such as squats and lunges (Figures 3, 4) target the hamstrings, gluteal muscles, and hip external rotators, muscle groups that work to counteract the hip adduction, hip internal rotation, and external tibial rotation associated with dynamic knee valgus. Exercises such as planks (Figure 5) and prone lifts also strengthen the hamstrings and gluteal muscles and improve trunk strength and stability.
FIGURE 3 Squats strengthen the gluteal muscles, and when done properly with hips, back, and tibias vertical, they equally engage the hamstrings and quadriceps.
FIGURE 4 In a forward lunge, athletes step forward into a lunge and then return to standing, keeping the back straight and not allowing the knees to pass the toes. Lunges are also performed in the sagittal plane (not shown) to evenly strengthen the quadriceps, hamstrings, and gluteal muscles; improve core stability; and minimize strength imbalance between legs.
FIGURE 5 Side planks strengthen the abdominal and gluteal muscles. The athlete holds the position for 30 seconds, keeping her body in a straight line.
Plyometrics are repetitive jumping exercises in which the targeted muscle group starts in the stretched position and then rapidly contracts with maximum force. This pairing of eccentric and concentric muscle contractions increases muscle power. Effective NMT programs incorporate plyometrics that gradually progress in difficulty from 2-legged takeoffs and landings (eg, squat jumps) to 1-legged takeoffs and landings (eg, hopping, or bounding, in place from 1 leg to the other), and from jumping in place to traveling jumps (eg, broad jump or single-leg hop for distance). Initially, the athlete performs as many repetitions as possible with good form for 20 seconds. As the program progresses and her strength improves, this interval increases to 30 seconds and then 40 seconds.
An important component of NMT programs is supervision by a qualified instructor or coach who has been specifically trained in recognizing dynamic knee valgus. The coach teaches athletes how to recognize and avoid this unsafe knee position by correcting athletes’ improper form and not allowing them to progress to more challenging exercises until they have demonstrated consistently proper form with less difficult exercises. Instructors use verbal cues such as “Don’t let your knees cave inward or come together” and “Land softly and quietly.” These cues remind athletes to contract their muscles to absorb the force while landing from a jump, rather than allowing their feet to just fall to the ground.
Optimal timing, frequency, and duration of training
Female athletes aged 15 to 18 years exhibit the largest decreases in ACL injury risk in response to NMT.31 Thus, the optimal time to begin NMT programs is during early adolescence. It appears that a minimum of 6 to 8 weeks of training is needed before neuromuscular changes are seen and athletic performance improves.40 The most effective NMT programs trained athletes for a minimum of twice per week for 6 weeks. A combination of preseason and in-season training was more effective than either preseason or in-season training alone.39 Some have proposed that the optimal method for implementing NMT programs may be through an integrative program incorporated into daily physical education classes, which would extend the benefits of such programs to individuals involved in recreational physical activities as well as competitive sports.31
Pediatricians caring for young people at increased risk of ACL injuries should counsel them on the potential benefits of NMT. Pediatricians can help athletes and their parents locate a qualified instructor in 1 of the following ways:
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DR DHARAMSI is a fellow in primary care sports medicine, McGaw Medical Center of Northwestern University, Chicago, Illinois. DR LABELLA is medical director, Institute for Sports Medicine, Ann and Robert H. Lurie Children’s Hospital of Chicago, and associate professor of pediatrics, Northwestern University Feinberg School of Medicine, Chicago. The authors have nothing to disclose in regard to affiliations with or financial interests in any organizations that may have an interest in any part of this article.