No sex differences in oxygen uptake or extraction kinetics in the moderate or heavy exercise intensity domains

Abstract The integrative response to exercise differs between sexes, with oxidative energy provision purported as a potential mechanism. However, there is a lack of systematic investigation into the kinetics of oxygen uptake ( O 2 ) and extraction (HHb+Mb) during exercise in both sexes. Sixteen young adults (8 females, age: 27±5 years) completed three experimental visits. Incremental exercise testing was performed to obtain lactate threshold and O2peak . Subsequent visits involved three six-minute cycling bouts at 80% of lactate threshold and one 30-minute bout at a work rate 30% between the lactate threshold and maximal ramp test power. Pulmonary gas exchange and near-infrared spectroscopy continuously sampled O 2 and HHb+Mb, respectively. The mean of six moderate and two heavy intensity transitions were modelled with mono-exponential curves to quantify the phase II response to exercise. Slow component amplitudes were also quantified for the heavy intensity domain. Males and females demonstrated similar relative O2peak values (46.2±6.6 vs 40.5±6.7 ml.kg - 1 .min -1 , p =0.111), with males achieving ∼30% greater power outputs ( p =0.002). In both the moderate and heavy intensity domains, the relative amplitude of the phase II transition was similar between sexes for O 2 ( p ≥0.179) and HHb+Mb ( p ≥0.193). Similarly, there were no sex differences in the time constants for O 2 ( p ≥0.385) or HHb+Mb ( p ≥0.274). In the heavy intensity domain, neither O 2 ( p ≥0.686) or HHb+Mb ( p ≥0.432) slow component amplitudes were different between sexes. The oxidative response to moderate and heavy intensity exercise did not differ between males and females, suggesting both sexes experience similar degrees of bioenergetic stress during intensity-matched exercise. New and Noteworthy This study demonstrated no sex differences in the bioenergetic response to moderate and heavy intensity cycling exercise. The change in oxygen uptake and deoxyhaemoglobin were modelled with mono-exponential curve fitting, which revealed that both sexes increased the rate of oxidative energy provision similarly. This provides insight into previously reported sex differences (e.g., fatigability) suggesting that the mechanisms are contractile rather than metabolic.