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SEX DIFFERENCES IN VOLUNTARY AND STIMULATED CRITICAL TORQUE: THE ROLE OF TWITCH CHARACTERISTICS AND MAXIMAL STRENGTH

Abstract

Previous research has demonstrated clear sex differences in fatigue rates likely related to differences in maximal strength between men and women. Little data exist examining sex differences in critical power/torque and whether differences in strength profiles and fiber-type between men and women might account for any observed differences. PURPOSE: The purpose of the present study was to examine relationships among muscle twitch properties, fatigue rate, and critical torque during both voluntary and electrically stimulated exercise. METHODS: Twenty individuals participated--10 men and 10 women. All were recreationally active. A voluntary critical torque (vCT) test was performed where participants performed 60 maximal voluntary isometric contractions at a 3sec:2sec work to rest cycle. On a separate day participants performed an electrically stimulated critical torque test (eCT) at a starting torque of 25% of their maximum voluntary contraction at a stimulation frequency of 100 Hz and a work to rest cycle of 2sec:2sec. Critical torque was calculated as a percentage of starting torque. Initial rate of fatigue (the slope of decline over the initial 10 contractions) was calculated, rate-of-force development and rate-of-force relaxation were also calculated from electrically stimulated doublet twitches as surrogates of fiber type. Independent t-tests were used to compare outcome measures between men and women and Pearson correlation coefficients were calculated to examine relationships among dependent measures. RESULTS: No sex differences were found for vCT (47.2 ± 10.5% vs 42.4 ± 14.3%; p = 0.40), but women exhibited a higher eCT compared to men (33.2 ± 7.9% vs 27.1 ± 7.8%; p < 0.001), a less steep slope of voluntary torque decline (0.51 ± 0.22 vs 0.93 ± 0.43; p < 0.001) and stimulated torque decline (0.49 ± 0.18 vs 0.79 ± 0.34; p = 0.01). No differences were found for rate-of-force development (p = 0.34) and rate-of-force relaxation (p = 0.22).A significant relationship was found between starting torque and critical torque in both voluntary (r = -0.51; p = 0.02) and stimulated ( r = -0.57 ; p = 0.009) exercise, rate of torque of development and critical torque for voluntary (r = -0.54; p = 0.02) and stimulated (r = -0.70; p < 0.001) and rate of torque relaxation and critical torque during voluntary (r = -0.45; p = 0.05) and stimulated (r = -0.49; p = 0.03). Additionally, the slope of torque decline was also correlated with starting torque (r = 0.69 for voluntary and r = 0.94 for stimulated p <0.001) and voluntary critical torque (r = -0.62; p = 0.003) and stimulated critical torque (r = -0.57; p = 0.008). CONCLUSIONS: The disparate finding of sex differences on voluntary and stimulated critical torque was unexpected and may be due to difference force required to induce ischemia. Twitch properties, which can serve as a non-invasive measure of relative fiber type were predictive of CT and initial rate of fatigue development.

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