The influence of sex, hemoglobin mass, and skeletal muscle characteristics on cycling critical power

Critical power (CP) represents an important threshold for exercise performance and fatiguability. We sought to determine the extent to which sex, hemoglobin mass (Hb ), and skeletal muscle characteristics influence CP. Prior to CP determination (i.e., 3-5 constant work rate trials to task failure), Hb and skeletal muscle oxidative capacity (τ) were measured and muscle biopsy samples were collected from 12 females and 12 males matched for aerobic fitness relative to fat-free mass (mean (SD); V̇O max: 59.2 (7.7) vs. 59.5 (7.1) mL·kg FFM ·min , respectively). Males had a significantly greater CP than females in absolute units (225 (28) vs. 170 (43) W; p=0.001) but not relative to body mass (3.0 (0.6) vs. 2.7 (0.6) W·kg BM ; p=0.267) or FFM (3.6 (0.7) vs. 3.7 (0.8) W·kg FFM ; p=0.622). Males had significantly greater W' (p ≤ 0.030) and greater Hb (p ≤ 0.016) than females, regardless of the normalization approach; however, there were no differences in mitochondrial protein content (p=0.375), τ (p=0.603), or MHC I proportionality (p=0.574) between males and females. Whether it was expressed in absolute or relative units, CP was positively correlated with Hb (0.444≤r≤0.695; p