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The stability of the tibiofemoral joint is maintained by passive (non-contractile) and dynamic (contractile) mechanisms. The passive mechanisms include the shape of the articular surfaces, the menisci, the ligaments and the joint capsule. The dynamic mechanisms consist of the muscle-tendon units that cross the joint, in particular, the quadriceps and hamstrings. The incidence of non-contact anterior cruciate ligament (ACL) injury is reported to be 6 to 8 times greater in females than males competing in the same activities. A number of intrinsic and extrinsic risk factors have been proposed to account for this gender difference in the incidence of ACL injuries. However, most of the proposed risk factors have arisen from uni-variate correlation studies based on relatively small samples. The purpose of this paper is to present a risk factor model for ACL injury based on a review of passive and dynamic stability mechanisms.