Hamstring weakness and activation deficits play a role in controlling dynamic valgus postures by resisting anterior and lateral tibial translation and tibial rotations (2). Studies have demonstrated that weak hamstrings result in quadriceps dominance secondary to increased knee extensor moments over knee flexor moments during tasks that require increased lower extremity demand, therefore increasing risk (60). Adolescent girls exhibited less hamstring strength in comparison to age- and size-matched boys, confirming need for earlier intervention (59).
Hamstring activation is modifiable through strengthening and neuromuscular training (4,79). Studies have demonstrated improved dynamic valgus control and prelanding and landing activation with jump landings with neuromuscular training (79). Eccentric loading of the hamstrings, such as partnered Russian hamstring curls, has been found to be more effective in improving hamstring to quad ratio than traditional concentric exercises (42,59).
Agility training programs that have incorporated unanticipated directional changes have demonstrated improved medial hamstring activation during pivoting as well as a significant reduction in knee valgus loading during planned and unplanned conditions (13,75).
Core weakness and proprioception have been associated with lateral trunk displacement, altered dynamic knee stability, and predicted knee injury risk (4,5,37,77–79). Effective programs should include core stability training, proprioceptive exercises, perturbations, and postural sway corrections. Core proprioceptive neuromuscular training has improved body sway; however, more research is needed secondary to the paucity in the literature on its effects on noncontact ACL rates.
Leg dominance may be secondary to asymmetrical muscle weakness, flexibility, and/or pain avoidance. Training should focus on each leg in isolation if dominance is noted. If pain is reported, then the child should be referred to a physician for proper evaluation.
During growth spurts, neuromuscular ability, coordination, and proprioception become imbalanced and contribute to risk of injury (37). Quadriceps and hamstring flexibility is critical to allow for sufficient knee and hip flexion during sports maneuvers to reduce ACL strain (37). In addition, proprioceptive training is vital to incorporate into one’s program to allow children to improve within their new center of mass. Multiple studies demonstrated ACL injury reduction with use of balance and proprioceptive training as part of a multifaceted program with strengthening and neuromuscular training (9,37).
Multifaceted programs that include strengthening, proprioception and neuromuscular training, plyometrics, and technique training have historically demonstrated the most success. However, limited research has been done with prepubescent children. Adapting a program specifically geared toward children may improve the ability to change neuromuscular risk factors for ACL injury and therefore reduce rate of injury entering into adolescence.
Historically, intervention strategies were focused on the adolescent athlete secondary to increased injuries incurred from 16 to 18 years of age. However, children between the ages of 10 and 12 years are demonstrating movement patterns associated with injury risk during sports-related movements (15) and injury rates are rising. Even though immediate injury risk may be less during this period, it serves as an ideal time to intervene and refine movement patterns to reduce future injury risk that tends to occur during adolescence (15).
The benefits of strength training, proprioceptive and neuromuscular training, flexibility, and plyometrics have been demonstrated in the literature to reduce injury rates in the adolescent athlete. In addition, it is evident that intervening at a young age is critical to the success in ACL injury prevention and may perhaps be the missing link. However, it is unclear whether this age group will respond to current adolescent ACL injury prevention programs that involve complex directions and may be beyond their level of comprehension and ability (14–16).
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