Sex differences in the maximal metabolic steady state of fitness-matched women and men.

Abstract

We tested the hypothesis that power at maximal metabolic steady state is similar between fitness-matched men and women. Eighteen participants (9 men and 9 women) performed a cycling graded exercise test for maximal oxygen consumption (V̇o_2max). Men and women were matched for V̇o_2max normalized to fat-free mass (FFM), which was 50.4 ± 4.7 mL·min^-1·kg·FFM^-1 and 52.1 ± 8.2 mL·min^-1·kg·FFM^-1, respectively (P = 0.62). Participants completed a muscle oxygenation (%SmO₂) zero-slope prediction trial and a 3-min all-out trial (3MT). The %SmO₂ zero-slope trials included three, 5-min cycling bouts (30-s rest) spanning intensity domains. Linear regression of trial work rate and %SmO₂ slope over the final 3 min established the work rate occurring at the predicted zero slope in %SmO₂. The 3MT required cycling all-out until the word "stop" without providing time elapsed. End test power (ETP) was calculated as the mean power output over the last 30 s and work above end test power (WEP) as the power-time integral above ETP. Independent of method, means ± SD absolute power at the maximal metabolic steady state was similar between fitness-matched women and men (P = 0.72), yet became higher in women when expressed relative to FFM (P = 0.02). Furthermore, V̇o₂ at the power associated with %SmO₂ zero-slope represented a significantly higher fraction of V̇o_2max for women compared with men (P = 0.03). Normalized WEP (W/kg·FFM) remained higher in men (P < 0.01). Although highly correlated (r = 0.88, P < 0.01), ETP was ∼8% higher than %SmO₂ zero-slope power (P = 0.03). Compared with fitness-matched men, women displayed higher FFM normalized power associated with the heavy-severe exercise domain boundary. When matched for fitness, women have a higher power associated with the heavy-severe domain boundary compared with men, when expressed relative to FFM. This exercise intensity also represents a higher fraction of maximal oxygen uptake for women; suggesting women can sustain a higher relative V̇o₂ compared with men. Power at the heavy-severe domain boundary, in both sexes, was lower using muscle oxygenation-derived slope methods compared with 3-min all-out determinations.NEW & NOTEWORTHY When matched for fitness, women have a higher power associated with the heavy-severe domain boundary compared with men, when expressed relative to FFM. This exercise intensity also represents a higher fraction of maximal oxygen uptake for women; suggesting women can sustain a higher relative V̇o₂ compared with men. Power at the heavy-severe domain boundary, in both sexes, was lower using muscle oxygenation derived slope methods compared with 3-min all-out determinations.