Fatigue: It's what happens if an athlete cuts corners on conditioning or starts a race too fast. It can come between an athlete and a new personal best.

The fatigued athletes were less likely to bend their knees on landing and were more likely to land with their knees turned inward.

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.

In a study presented last month at the World Congress on Injury Prevention in Tromso, Norway, the Michigan researchers found that 18 female recreational athletes were more likely when fatigued to land from a jump in a way that would predispose them to an ACL injury. Specifically, the fatigued athletes were less likely to bend their knees on landing and were more likely to land with their knees turned inward; torques produced at the knee were also higher when the athletes were fatigued.

The athletes' landing patterns were also studied under anticipated and unanticipated conditions. For the unanticipated landings, the athlete did not know whether to land on her left or right leg until given a light cue at the last moment. The detrimental effects of fatigue on landing patterns were exacerbated in the unanticipated condition, suggesting that fatigued athletes are particularly ill-equipped to respond to sudden developments and thus even more at risk of injury.

Perhaps the study's most interesting finding was that, although the fatigue protocol focused on only one limb in each athlete, the fatigue-induced changes in landing patterns were similar in both limbs. This supports the authors' theory that the type of fatigue experienced by athletes is not limited to the muscles themselves but rather affects the central nervous system, limiting the brain's capacity for motor control.

"In a fatigued state, athletes are making worse decisions, and information isn't being communicated as effectively to the muscles," said Scott G. McLean, PhD, assistant professor of athletic training and movement science and director of the Injury Biomechanics Laboratory at the University of Michigan, who presented the findings in Tromso. "The question now is whether we can train people to better adapt to those fatigue effects."