Sex differences in human skeletal muscle fiber types and the influence of age, physical activity, and muscle group: A systematic review and meta‐analysis
Jessica J. James, Maddison L. Mellow, Elizabeth P. Bueckers, Savannah B. Gutsch, David J. Wrucke, Andrew G. Pearson, Ashleigh E. Smith, Sandra K. Hunter
Abstract
Abstract To understand the sex differences in human skeletal muscle fibers, we determined whether sex differences in fiber cross‐sectional area (CSA), fiber type distribution, and proportional area remained after controlling for age, physical activity level, muscle group, and analysis technique. Meta‐analysis was performed on 6222 unique participants (Males, M = 3501; Females, F = 2721) (>18 years and free of disease) extracted from 156 studies. A random‐effects meta‐analysis was used to determine the main effect of sex, and subgroup analyses were performed to determine the influence of age, physical activity level, muscle group biopsied, and analysis technique. Males had greater type I CSA (M = 4936 ± 1250 μm 2 ; F = 4151 ± 1074 μm 2 ; p < 0.001), type II CSA (M = 5272 ± 1950 μm 2 ; F = 3483 ± 1309 μm 2 ; p < 0.001), type II distribution (M = 51.6 ± 14.6%; F = 48.3 ± 13.0%; p < 0.001), and type II proportional area (M = 55.0 ± 14.4%; F = 47.9 ± 13.1%; p < 0.001) than females. Conversely, females had greater type I distribution (F = 51.4 ± 12.1%; M = 48.3 ± 13.3%; p = 0.01) and type I proportional area (F = 51.8 ± 12.4%; M = 44.9 ± 13.2%; p < 0.001) than males. Sex differences were moderated by muscle group biopsied in type I and II proportional area and by age in type I and II fiber type distribution but remained in all other subgroup analyses. In healthy adults, males have larger type I and type II CSA, type II fiber type distribution, and type II proportional area than females, while females have greater type I fiber type distribution and type I proportional area than males. Sex differences generally remained regardless of age, physical activity level, muscle group, and analysis technique, indicating inherent biological sex differences in muscle fiber composition of whole skeletal muscle.