Sex differences in endurance capacity and metabolic response to prolonged, heavy exercise

In order to test for possible sex differences in endurance capacity, groups of young, physically active women (n=6) and men (n=7) performed bicycle ergometer exercise at 80% and 90% of their maximal oxygen uptakes ( \(\dot V_{{\text{O}}_{\text{2}} {\text{max}}}\) ). The groups were matched for age and physical activity habits. At 80% \(\dot V_{{\text{O}}_{\text{2}} {\text{max}}}\) the women performed significantly longer (P<0.05), 53.8±12.7 min vs 36.8±12.2 min, respectively (means ± SD). Mid-exercise and terminal respiratory exchange ratio (R) values were significantly lower in women, suggesting a later occurrence of muscle glycogen depletion as a factor in their enhanced endurance. At 90% \(\dot V_{{\text{O}}_{\text{2}} {\text{max}}}\) the endurance times were similar for men and women, 21.2±10.3 min and 22.0±5.0 min, respectively. The blood lactate levels reached in these experiments were only marginally lower (mean differences 1.5 to 2 mmol·l⁻¹) than those obtained at \(\dot V_{{\text{O}}_{\text{2}} {\text{max}}}\) , suggesting high lactate levels as a factor in exhaustion. The changes in body weight during the 80% experiments and the degree of hemoconcentration were not significantly different between men and women.