But research from the University of Michigan also suggests that fatigued athletes are far more vulnerable to an anterior cruciate ligament (ACL) tear, one of the most devastating sports injuries. Not only are tired muscles and joints unable to perform properly, but mental fatigue can delay reaction times — which can be critical when defending a quick-footed opponent or navigating a stretch of unfamiliar terrain. This is particularly important for female athletes, whose risk of ACL injury is up to eight times that of their male counterparts.The fatigued athletes were less likely to bend their knees on landing and were more likely to land with their knees turned inward.