Sex differences in the power-duration relationship for dynamic knee extension are largely explained by muscle size

Although males generally outperform females in absolute exercise tasks, it is unclear whether these differences persist for small muscle mass exercise when accounting for muscle size and anatomical factors. The purpose of this study was to compare the power-duration relationship for dynamic knee extension exercise between healthy males (n = 12) and females (n = 12) matched for maximal aerobic fitness relative to fat-free mass (V̇o_2max: 59.5 ± 7.9 vs. 59.2 ± 6.0 mL·kgFFM^-1·min^-1; P = 0.921). Participants completed four constant work rate tests to measure critical power (CP) and the curvature constant (W'). Using magnetic resonance imaging (MRI), quadriceps volume was assessed to estimate and control for muscle mass, and the patellar tendon moment arm (PTMA) was measured to estimate quadriceps force. Males demonstrated a significantly greater CP (P = 0.003), CP torque (P < 0.001), CP quadriceps force (P = 0.003), and W' (P = 0.031) compared with females. No sex difference in CP measures remained after controlling for quadriceps mass (P > 0.05). An interaction between sex and quadriceps mass regarding W' (P = 0.036) revealed a positive association in males (P = 0.001) but not in females (P = 0.926). After adjusting for quadriceps mass, whole body and single-leg aerobic fitness were related to CP (P < 0.05). Maximal quadriceps force was related to W' (P = 0.044), but strength did not appear to influence CP independently of quadriceps mass (P > 0.05). Overall, sex differences in absolute CP were largely explained by muscle size as opposed to an innate difference between male and female muscle, and variability in CP was not independently related to strength or PTMA.NEW & NOTEWORTHY In males and females matched for aerobic fitness [maximal oxygen uptake normalized to fat-free mass (FFM)], dynamic knee extension critical power (CP) was greater in males, but after adjusting for differences in quadriceps muscle size, CP was similar between sexes. These findings indicate that sex differences in small muscle mass performance result from differences in muscle size, not innate differences in muscle endurance capacity.