Sex differences in valgus knee angle during a single-leg drop jump.

CONTEXT: Sex differences in lower extremity landing mechanics and muscle activation have been identified as potential causative factors leading to the increased incidence of anterior cruciate ligament injuries in female athletes. Valgus knee alignment places greater strain on the anterior cruciate ligament than a more neutral alignment. Gluteus medius (GM) activation may stabilize the leg and pelvis during landing, limiting valgus knee motion and potentially preventing anterior cruciate ligament injury. OBJECTIVE: To determine if frontal-plane knee angle and GM activation differ between the sexes at initial contact and maximal knee flexion during a single-leg drop landing. DESIGN: Between-groups design. SETTING: Motion analysis laboratory. PATIENTS OR OTHER PARTICIPANTS: Thirty-two healthy subjects between the ages of 18 and 30 years. INTERVENTION(S): The independent variables were sex (male or female) and position (initial contact or maximal knee flexion). MAIN OUTCOME MEASURE(S): Frontal-plane knee angle and GM average root mean square (aRMS) amplitude. RESULTS: At initial contact, women landed in knee valgus and men landed in knee varus (P < .025). At maximal knee flexion, both men and women were in a position of knee varus, but the magnitude of varus was less in women than in men (P < .025). The GM aRMS amplitude was greater at maximal knee flexion than at initial contact (P < .025); however, male GM aRMS did not differ from female GM aRMS amplitude at either position (P > .025). CONCLUSIONS: Women tended to land in more knee valgus before and at impact than men. The GM muscle activation did not differ between the sexes and, thus, does not appear to be responsible for the sex differences in knee valgus. The excessive valgus knee angles displayed in women may help to explain the sex disparity in anterior cruciate ligament injury.