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Non-contact anterior cruciate ligament injury is one of the most commonly seen injuries in sports with an estimated total cost for treatment over $2 billion per year. To prevent non-contact ACL injuries, several possible risk factors have been proposed. No studies to date, however, have shown conclusive evidence that any of the proposed risk factors are indeed risk factors for sustaining non-contact ACL injuries. In this study, a sagittal plane knee model was developed to enhance the understanding of the effects of sagittal plane biomechanics on the ACL loading. Knee geometry data were obtained from x-ray films of 10 male and 10 female collegiate aged recreational athletes. The patellar tendon-tibial shaft angle, patella-patellar tendon angle, quadriceps tendon-patella angle, and hamstring tendon-tibial shaft angle were expressed as functions of knee flexion angle, and incorporated into the sagittal plane knee model to estimate the ACL loading. In vivo lower extremity kinematic and kinetic data of a stop-jump task collected from 80 college aged recreational athletes were used for computer simulation with the sagittal plane knee model. A stochastic biomechanical model was developed and used to determine the risk and risk factors of sustaining non-contact ACL injuries during landing of the stop-jump task. The results of the computer simulation with sagittal plane inverse dynamic knee model showed that the lower extremity sagittal plane biomechanics simulated did affect the ACL loading. The results of the stochastic biomechanical model presented that females had higher risk of non-contact ACL injuries than males did. The gender difference in the risk of non-contact ACL injuries was affected by the differences in the lower extremity motion patterns, ACL ultimate load, and knee anatomy between genders. Biomechanical characteristics during the stop-jump task were significantly different between injured and uninjured jumps or between females and males. During the stop-jump task, the isolated sagittal plane loading alone can injure the ACL while the isolated non-sagittal plane loading alone may not. Knee flexion angle and peak posterior ground reaction force are two important risk factors for the gender difference in the risk of non-contact ACL injuries during the stop-jump task.