Sex difference in maximal oxygen uptake

Ten men and 11 women were studied to determine the effect of experimentally equating haemoglobin concentration ([Hb]) on the sex difference in maximal oxygen uptake ( \(\dot V_{O_{2_{\max } } } \) ). \(\dot V_{O_{2_{\max } } } \) was measured on a cycle ergometer using a continuous, load-incremented protocol. The men were studied under two conditions: 1) with normal [Hb] (153 g · L⁻¹) and 2) two days following withdrawal of blood, which reduced their mean [Hb] to exactly equal the mean of the women (134 g· L⁻¹). Prior to blood withdrawal, \(\dot V_{O_{2_{\max } } } \) expressed in L · min⁻¹ and relative to body weight and ride time on the cycle ergometer test were greater (p<.01) in men by 1.11 L · min⁻¹ (47%), 4.8 ml · kg⁻¹ min⁻¹ (11.5%) and 5.9 min (67%), respectively, whereas \(\dot V_{O_{2_{\max } } } \) expressed relative to fat-free weight (FFW) was not significantly different. Equalizing [Hb] reduced (p<.01) the mean \(\dot V_{O_{2_{\max } } } \) of the men by 0.26 L · min⁻¹ (7.5%), 3.2 ml · kg⁻¹ min⁻¹ (6.9%) or 4.1 ml · kg FFW⁻¹ min⁻¹ (7.7%), and ride time by 0.7 min (4.8%). Equalizing [Hb] reduced the sex difference for \(\dot V_{O_{2_{\max } } } \) less than predicted from proportional changes in the oxygen content of the arterial blood and arteriovenous oxygen content difference during maximal exercise. It was concluded that the sex difference in [Hb] accounts for a significant, but relatively small portion of the sex difference in \(\dot V_{O_{2_{\max } } } \) (L · min⁻¹). Other factors such as the dimensions of the oxygen transport system and musculature are of greater importance.