Injuries of the anterior cruciate ligament (ACL), located in the knee, are typically thought to be caused by acute traumatic events, such as sudden twists. New work analyzing an animal model of ACLs suggests that such injuries can also occur as a result of chronic overuse, specifically due to a reduced ability to repair microtraumas associated with overuse. Importantly, the team said, females also are less able to heal from these microtraumas than males, which may explain why females are two to eight times more likely to tear their ACL ligaments than males.