Sex differences in contractile properties and fatigue resistance of human skeletal muscle.

To explore the cause of higher skeletal muscle fatigue resistance in women than men, we used electrically evoked contractions (1 s on, 1 s off, 30 Hz, 2 min), which circumvent motivational bias and allow examination of contractile properties. We compared 29 men [26.5 (7.0) years old; mean (s.d.)] with 35 women [25.4 (7.6) years old]. Strength of the quadriceps muscle was higher in men than women (P < 0.001). The lower maximal rate of relaxation in women (P = 0.002) indicates that their muscles were slower than those of men. The torque declined less in women than in men [37.7 (10.7) versus 29.9 (10.0)%; P = 0.002], and was not related to muscle strength or size, as determined with magnetic resonance imaging. The sex difference in fatigability was also seen when the circulation to the leg was occluded [torque declined 76.9 (10.8) versus 59.5 (16.9)% in men versus women, respectively; P = 0.008]. The maximal rate of relaxation correlated with the fatigability of the muscle under all conditions (correlations ranging from 0.34 to 0.51, P < 0.02). We conclude that the sex-related difference in skeletal muscle fatigue resistance is not explicable by differences in motivation, muscle size, oxidative capacity and/or blood flow between sexes, but might be related to differences in fibre type composition.