Contraction intensity and sex differences in knee-extensor fatigability.

Females are less fatigable than males during isometric contractions across various muscles and intensities. However, sex differences in knee-extensor fatigability remain relatively unexplored.

PURPOSE: To determine the sex difference in performance fatigability for intermittent, isometric contractions of the knee-extensor muscles.

METHODS: Eighteen participants (10 males, 8 females) performed intermittent, isometric, knee-extensor contractions at 30% of their maximal voluntary force (MVC) for 30min and in a separate session at 50% MVC until task-failure. During both fatiguing protocols a MVC was performed every 60s and electromyography (EMG) was recorded during all contractions.

RESULTS: At task completion males had a larger reduction in MVC force for the 30% MVC task (-32±15% vs. -15±16%, P=0.042) and the 50% MVC task (-34±8% vs. -24±1%, P=0.045). Furthermore, for the 50% MVC task, females had a longer task duration (937±525svs. 397±153s, P=0.007). The rise in EMG activity and force fluctuations were more rapid for the males than females (P<0.05). When participants were matched for strength post hoc (n=10), a sex difference in fatigability for both tasks was still evident.

CONCLUSIONS: Females were less fatigable than males during intermittent, isometric, knee-extensor contractions at moderate relative forces and this difference was independent of strength.