Significant primary effects of fatigue were observed in the gluteus maximus muscle

1 2 Purpose: To determine how sex and fatigue affect lower limb kinematics, kinetics, and 3 muscle activity during unanticipated side-step cutting. 4 Methods: Twenty-three physically active subjects (men: 11, women: 12) performed 10 5 successful trials of cutting maneuvers each to either side under unanticipated conditions 6 in response to 2 light emitting diodes before and after fatigue conditions. Data were 7 analyzed and compared regarding sex and fatigue conditions using 2-way repeated 8 measures analysis of variance. 9 Results: After fatigue-inducing exercise, women demonstrated larger impulses of 10 ground reaction force (IGRF) during the first 50 ms (2.4 ± 0.8 vs. 2.1± 0.9, P < 0.05) 11 than did men. Significant primary effects of sex indicated that women showed a smaller 12 hip flexion angle at initial contact (40.4 ± 6.9° vs. 49.7 ± 9.1°, P < 0.05) and at 13 maximum flexion angle (41.3 ± 7.7° vs. 51.4 ± 9.0°, P < 0.05) compared with men. 14 Significant primary effects of fatigue were observed in the gluteus maximus muscle 15 during 50 ms before initial contact (+21.5 ± 48.3%, P < 0.05) and in the 16 semimembranosus muscle during 50 ms before initial contact (-6.2 ± 20.1%, P < 0.05) 17 and the first 50 ms of side-step cutting (-7.9 ± 26.6%, P < 0.05). 18 Conclusions: Our results suggest that sex differences, especially larger IGRF in a 19 fatigue state combined with less hip flexion angle, leads to women having a higher risk 20 for ACL injury. These findings may contribute to understanding the underlying 21 mechanism of injury and development of preventive exercises against ACL injury. 22 Level of evidence: Prospective comparative study, Level II. 23