Sex-specific knee joint strength and muscular endurance profiles in healthy adolescents

We examined sex-differences in isometric and dynamic knee strength and muscular endurance among healthy adolescent males and females, and established baseline metrics to better inform ACL rehabilitation protocols for this population. One hundred healthy adolescents completed isometric knee extension and flexion contractions on an isokinetic dynamometer, and an endurance task involving forty repetitions of maximal concentric-concentric knee extension and flexion contractions at 90°/s through an 85° range of motion. Peak torques were extracted from each limb and used to calculate hamstrings-quadriceps torque ratios and fatigue indices. Males exhibited a lower fatigue index than females for knee flexion and no difference for knee extension. Males showed higher isometric knee extension and flexion torques, higher isokinetic knee flexion torque at the end of the endurance task and higher knee extension torque at the start and end of the endurance task. Limb symmetry differences were also more evident during the endurance task compared to the isometric testing. These findings suggest that adolescent males and females exhibit distinct force generation abilities and that their sex-dependent endurance characteristics differ from those of adults. The inclusion of an endurance task in rehabilitation protocols may also provide additional insight that may not be evident via isometric testing.