PROJECT SUMMARY The anterior cruciate ligament (ACL) is one of the most commonly injured ligaments of the knee, with female athletes sustaining ACL injuries at a 2-8 fold greater rate compared to male athletes. An ACL injury can be devastating and significantly increases the athlete's risk for osteoarthritis long term. While many advances have been made in terms of surgical and rehabilitation treatments for ACL injured patients, long term outcome studies show that these patients are at a high risk for developing knee osteoarthritis 10-15 years after ACL injury regardless of the treatment. Currently, the mechanism of non-contact ACL injury is not well understood. In order to design successful ACL injury prevention programs and address the high rates of ACL injuries in the female athlete population, a better understanding of the non-contact ACL injury mechanism must be established. Both biomechanical data and video analyses indicate that increased abduction loads in the lower extremities may be associated with increased ACL strain and risk of injury. However, the medial collateral ligament (MCL) is considered to be the primary restraint against abduction stress in the knee joint. Clinicians and biomechanists are unable to explain why ACL ruptures without MCL injuries are significantly more common than combined ACL/MCL injuries. We hypothesize that coupled abduction and anterior knee joint loads near full knee extension will lead to disproportional increases in ACL strain relative to MCL strain and lead to ACL failure loads without concomitant MCL failure loads in female athletes. We propose to use a unique blend of the current methods used to investigate ACL injury mechanisms in order to gain an in depth understanding of knee ligament biomechanics during high knee loading conditions. Specifically, we aim to determine the mechanical responses of the ACL and MCL to loads using cadaveric testing, computer mathematical modeling, and three-dimensional motion analysis. Identifying ACL injury mechanisms may help us develop ACL injury prevention programs that would allow many athletes to receive the health benefits of sports participation and avoid the long term sequelae of disability associated with knee osteoarthritis.