Anterior cruciate ligament (ACL) injuries are common in sports. Most ACL injuries are noncontact in nature and frequently occur in certain athletic tasks. In sports such as soccer, basketball and volleyball, female athletes have higher risk for ACL injuries than their male counterparts. Previous studies on the proposed risk factors for noncontact ACL injuries focused mainly on nonmodifiable static factors that are remote from the injury events, without consideration of movements. Little convincing evidence, theoretical or experimental, has demonstrated the association between these static factors and the risk of noncontact ACL injuries. Female athletes have altered knee motion patterns in athletic tasks that place the ACL at risk. These altered motion patterns by female athletes tend to increase the load on the ACL, and may be a major contributor to the elevated frequency of ACL injuries in female athletes.