Sex Differences in Mechanisms of Recovery after Isometric and Dynamic Fatiguing Tasks

Purpose The purpose of this study was to determine whether supraspinal mechanisms contribute to the sex difference in fatigability during and recovery from a dynamic and isometric fatiguing task with the knee extensors. Methods Transcranial magnetic stimulation and electrical stimulation were used to determine voluntary activation and contractile properties of the knee extensors in 14 men and 17 women (20.8 1.9 yr) after a 1) 60-s sustained, maximal voluntary isometric contraction (MVIC), and 2) dynamic fatiguing task involving 120 maximal voluntary concentric contractions with a 20% MVIC load. Results There were no differences between men and women in the reduction of maximal torque during the sustained MVIC (54.4% +/- 18.9% vs 55.9% +/- 11.2%, P = 0.49) or in the decrease in power during the dynamic fatiguing task (14.7% +/- 20.1% vs 14.2% +/- 18.5%, P = 0.92). However, MVIC torque recovered more quickly for women than men after the sustained MVIC and the dynamic task (P < 0.05). The transcranial magnetic stimulation-elicited superimposed twitch was larger for men than for women during the sustained MVIC and in recovery (immediately post, R0.1: 4.7% +/- 3.3% vs 2.4% +/- 1.9% MVIC; P = 0.02), with no sex difference after the dynamic task (P = 0.35). The reduction in resting twitch amplitude was larger for men than for women immediately after the dynamic task (37% +/- 22% vs 23% +/- 18%; P = 0.016) with no sex difference after the sustained MVIC (64% +/- 16% vs 67% +/- 11%; P = 0.46). Conclusions Supraspinal fatigue contributed to fatigability of the knee extensors more for men than for women after a maximal isometric task, whereas contractile mechanisms explained the sex difference in torque recovery after the fast-velocity dynamic task. The mechanisms for the sex difference in fatigability are task dependent.