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Muscle coordination during single-leg landing: Implications for ACL injuries

INTRODUCTION Knee injuries, particularly ruptures of the anterior cruciate ligament (ACL), are common among young athletes, and are widely accepted to be multi-factorial [1]. The quadriceps muscles (Q), hamstring muscles (H), and knee ligaments are thought to stabilize the knee during high risk movements; however, the specific role of muscle forces with regard to ACL injury remains unknown. Single-leg landing is one activity where ACL injuries routinely occur, and the timing of such injuries is hypothesized to be at the time of peak ground reaction force (GRF) [2]. Thus, an understanding of muscle cocontraction at peak GRF may lead to improved training methods for injury prevention. Hamstrings activation protects the ACL during landing [1], while quadriceps activation may endanger the ACL through an anterior force on the tibia. Despite this, the quadriceps muscles are needed to decelerate the body during landing and stabilize the knee joint. Li et al. [3] showed that quadriceps and hamstrings co-contraction effectively reduce ACL loading. It is nearly impossible to observe ACL injuries during in vivo testing; however, the combination of motion analysis and musculoskeletal modeling techniques can provide estimates of muscle forces during a landing maneuver. The Q/H force ratio at the time of peak GRF can show the net effect of both Q and H forces at the most likely instance of ACL injury. A Q/H force ratio that is substantially greater than one may indicate reduced ACL protection. We hypothesized that for increased landing heights, the knee muscles would be coordinated to increase protection of the ACL; that is, the Q/H ratio would be closer to one.