Sex differences in isolated skeletal muscle force production: a spectrum of responses

Abstract

Whether sex differences in skeletal muscle microvascular responses differ in response to muscle contraction is currently unknown. To determine vascular responses to muscle contraction in vivo we use intravital microscopy using thin skeletal muscle models of mixed fibre types (i.e. hamster retractor muscle). However, to effectively quantify this relationship in vivo, the initial muscle contraction stimulus must be constant across the sexes and thus, we must first determine whether there are sex differences in skeletal muscle force generation. We hypothesized that sexual dimorphisms in force production would not be apparent in muscles of any fibre type composition. To test this hypothesis, we used skeletal muscles with unique fibre type characteristics and oxidative capacities. Mouse soleus (SOL: primarily type 1 >75%), extensor digitorum longus (EDL: primarily type IIB ~60%), and costal diaphragm (DIA: primarily type IIA ~55%) muscles were isolated from adult male and female CD-1 mice. The retractor muscle (RET: primarily type IIX ~58%) was also isolated from adult male and female Syrian hamsters. Muscles were electrically stimulated in vitro and force production was quantified at optimal length during a force-frequency test (1-120Hz) and twitch (1Hz) contractions. In mice, females generated greater force during the force-frequency test in SOL (10-120Hz) and EDL (30-120Hz) and generated greater twitch forces compared to males (SOL: female 34.7 ± 2.4 mN/mm 2 , n=21 vs male 27.8 ± 2.0 mN/mm 2 , n=17; EDL: female 41.2 ± 2.2 mN/mm 2 , n=35 vs male 32.5 ± 2.1 mN/mm 2 , n=29). Females generated greater force during the force frequency test in DIA at 30Hz only and no significant sex differences were observed in DIA twitch force (female 0.24 ± 0.02 g/mg, n=19 vs male 0.20 ± 0.02 g/mg, n=21). In hamster RET no significant sex differences in force were observed during the force frequency test or during the twitch contraction (female 0.13 ± 0.01 g/mg, n=19 vs male 0.14 ± 0.02 g/mg, n=20). Collectively, these findings demonstrate that sex differences in force production were observed in muscles primarily comprised of slow oxidative (type 1) and fast glycolytic (type IIB) fibres but not in muscles primarily comprised of intermediate fibres (type IIA and type IIX). These data show that on a spectrum of muscle fibres from slow oxidative to fast glycolytic, sex differences are most apparent on the extremes of the spectrum, whereas sexual dimorphisms in the middle of the spectrum are not apparent. This work is supported by NSERC. NSERC This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.