abstract
- There are reports that females compared to males display increased skeletal muscle oxidative capacity in resting mixed-muscle fiber samples from the vastus lateralis, 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 if the oxidative capacity of type I and/or II fibers differ 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 (n=14; 23±5yr; 23.3±3.2kg/m2) and males (n=13; 23±4yr; 23.1±2.4kg/m2) were analyzed via immunofluorescence staining. There were no sex differences in indices of capillarization (all p>0.06) or mitochondrial content (all p>0.42) 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; p<0.001) but not females (335±77 vs. 329±48 capillaries/mm2; p=0.76), owing to greater cross-sectional area (CSA) of type II vs. type I fibers in males only (males p=0.03; females p=0.44). Females compared to males also displayed greater proportionate area of type I fibers (44±12 vs. 31±4%; p=0.03) and smaller CSA of type IIx fibers (3033±902 vs. 5573±1352 um2; p=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.