Over 100,000 anterior cruciate ligament (ACL) injuries occur annually in the United States [1]. Of these, 70% are classified as non-contact, many of which occur subsequent to a landing from a jump [2]. While most agree that quadriceps (Q) and hamstrings (H) have a significant contribution in knee biomechanics, the role of quadriceps and hamstrings muscle loads and their ratio (Q/H) in ACL injury remains controversial. Understanding muscle recruitment in high risk activities may improve our knowledge of ACL injury mechanisms. Such insight may improve current prevention strategies to decrease the risk of ACL injury and damage to secondary anatomical structures, all of which may in turn minimize associated posttraumatic knee osteoarthritis. As in vivo quantification of muscle loads remains challenging, especially under dynamic conditions, validated finite element (FE) models of the knee can be used to characterize the role of muscle loads in ACL injury. FE analysis has provided considerable insight into knee joint biomechanics, including ligament function, ligament reconstruction technique and implant design. This study utilized a validated FE model of the knee joint to study the effects of quadriceps to hamstrings ratio (Q/H) on ACL strain during a simulated landing from a jump. We hypothesized that both the ratio and magnitude of muscle loads are critical determinants of ACL loading. Further, a threshold may be reached as the magnitude of quadriceps load exceeds hamstrings load.

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