Sex differences in the oxygen delivery, extraction, and uptake during moderate-walking exercise transition

Previous studies in children and older adults demonstrated faster oxygen uptake ((V)over dotO(2)) kinetics in males compared with females, but young healthy adults have not been studied. We hypothesized that young men would have faster aerobic system dynamics in response to the onset of exercise than women. Interactions between oxygen supply and utilization were characterized by the dynamics of (V)over dotO(2), deoxyhemoglobin (HHb), tissue saturation index (TSI), cardiac output ((Q)over dot), and calculated arteriovenous O-2 difference (a-vO(2diff)) in women and men. Eighteen healthy active young women and men (9 of each sex) with similar aerobic fitness levels volunteered for this study. Participants performed an incremental cardiopulmonary treadmill exercise test and 3 moderate-intensity treadmill exercise tests (at 80% (V)Over dotO(2) of gas exchange threshold). Data related to the moderate exercise were submitted to exponential data modelling to obtain parameters related to the aerobic system dynamics. The time constants ofV(O)ver dotO(2), a-vO(2diff), HHb, and TSI (30 +/- 6, 29 +/- 1, 16 +/- 1, and 15 +/- 2 s, respectively) in women were statistically (p < 0.05) faster than the time constants in men (42 +/- 10, 49 +/- 21, 19 +/- 3, and 20 +/- 4 s, respectively). Although (Q)Over dot dynamics were not statistically different (p = 0.06) between groups, there was a trend to slower (Q)Over dot dynamics in men corresponding with the slower (V) Over dotO(2) kinetics. These results indicated that the peripheral and pulmonary oxygen extraction dynamics were remarkably faster in women. Thus, contrary to the hypothesis, (V) Over dotO(2) dynamics measured at the mouth at the onset of submaximal treadmill walking were faster in women compared with men.