The influence of sex on fiber-specific indices of oxidative capacity in human skeletal muscle

There are reports that females compared to males display increased skeletal muscle oxidative capacity in resting mixed-muscle fiber samples from the <i>vastus lateralis</i>, including markers of mitochondrial content and capillarization. Given that sex comparisons at the mixed-fiber level may be explained by differences in muscle fiber type between males and females, it remains unclear whether the oxidative capacity of type I and/or II fibers differs between sexes. The purpose of this study was to evaluate the influence of sex on fiber-specific indices of mitochondrial content and capillarization in healthy untrained males and females. Resting skeletal muscle samples from eumenorrheic females (<i>n</i> = 14; 23 ± 5 yr; 23.3 ± 3.2 kg/m²) and males (<i>n</i> = 13; 23 ± 4 yr; 23.1 ± 2.4 kg/m²) were analyzed via immunofluorescence staining. There were no sex differences in indices of capillarization (all <i>P</i> > 0.05) or mitochondrial content (all <i>P</i> > 0.05) in type I or type II muscle fibers. However, we observed lower capillary density in type II vs. type I muscle fibers in males (280 ± 66 vs. 364 ± 88 capillaries/mm²; <i>P</i> < 0.001) but not females (335 ± 77 vs. 329 ± 48 capillaries/mm²; <i>P</i> = 0.76), owing to greater cross-sectional area (CSA) of type II vs. type I fibers in males only (males <i>P</i> = 0.03; females <i>P</i> = 0.44). Females compared to males also displayed greater proportionate area of type I fibers (44 ± 12% vs. 31 ± 14%; <i>P</i> = 0.03) and smaller CSA of type IIx fibers (3,033 ± 902 vs. 5,573 ± 1,352 μm²; <i>P</i> = 0.002). Our results suggest that while muscle fiber type composition and size differ between males and females, there are no sex differences in mitochondrial content and capillarization of type I or II muscle fibers in untrained adults.<b>NEW & NOTEWORTHY</b> Research suggests that skeletal muscle oxidative capacity in mixed-fiber muscle homogenates is greater in females than in males. In healthy, untrained individuals, we demonstrate by fiber-specific immunofluorescence that females have a greater proportionate area of type I muscle fibers but no difference in mitochondrial content or capillarization of type I or II fibers compared to males. These findings suggest that although females display a more oxidative fiber type composition, sex does not influence muscle fiber-specific oxidative capacity.