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Female athletes who participate in sports involving jumping and cutting maneuvers are up to eight times more likely to sustain a rupture of the anterior cruciate ligament than are men participating in the same sports. We tested the hypothesis that healthy young women are able to volitionally increase the apparent torsional stiffness of the knee, by maximally activating the knee muscles, significantly less than are size-matched men participating in the same type of sport.Twenty-four NCAA (National Collegiate Athletic Association) Division-I athletes (twelve men and twelve women) competing in sports associated with a high risk of injury to the anterior cruciate ligament (basketball, volleyball, and soccer) were compared with twenty-eight collegiate endurance athletes (fourteen men and fourteen women) participating in sports associated with a low risk of such injuries (bicycling, crew, and running). Male and female pairs were matched for age, height, weight, body mass index, shoe size, and activity level. Testing was performed with a weighted pendulum that applied a medially directed 80-N impulse force to the lateral aspect of the right forefoot. The resulting internal rotation of the leg was measured optically, to the nearest 0.25°, at 30° and 60° of knee flexion, both with and without maximal activation of the knee muscles.Maximal rotations of the leg were greater in women than in men in both the passive and the active muscle state (16% and 27% greater [p = 0.01 and p = 0.02], respectively). Moreover, female athletes exhibited a significantly (18%) smaller volitional increase in apparent torsional stiffness of the knee under internal rotation loading than did the matched male athletes (p = 0.014); this was particularly the case for those who participated in sports involving jumping and pivoting maneuvers (42% difference between genders, p = 0.001).The collegiate female athletes involved in high-risk sports exhibited less muscular protection of the knee ligaments during external loading of the knee than did size and sport-matched male athletes.Improving active muscle protection of the knee during training and rehabilitation might help to decrease rates of knee injury.