Maximal cardiorespiratory responses to one- and two-legged cycling during acute and long-term exposure to 4300 meters altitude

During exposure to altitudes greater than about 2200 m, maximal oxygen uptake ( $\dot V_{O_{2max} } $ ) is immediately diminished in proportion to the reduction in the partial pressure of oxygen in the inspired air. If the exposure lasts longer than a couple of days, an increase in arterial oxygen content (CaO₂), due to a hemoconcentration and an increase in arterial oxygen saturation, occurs. However, there is also a reduction in maximal cardiac output ( $\dot Q_{\max }$ ) at altitude which offsets the increase in CaO₂ and, therefore, $\dot V_{O_{2max} } $ does not improve. The purpose of this investigation was to study the contribution of the increase in CaO₂ to the working muscles without the potentially confounding problem of a reduced $\dot Q_{\max }$ . The approach used was to have seven male subjects (aged 17 to 24 years) perform one- and two-legged $\dot V_{O_{2max} } $ tests on a cycle ergometer at sea level (SL, PIO₂ = 159 Torr), after 1 h at 4300 m simulated altitude (SA, PIO₂ = 94 Torr) and during two weeks of residence on the summit of Pikes Peak, CO. (pP, 4300 m, PIO₂ = 94 Torr). Cardiac output limits maximal performance during two-legged cycling but does not limit performance during one-legged cycling. During the study, CaO₂ changed from 189±3 (mean ±SE) at SL to 161±4 ml·L⁻¹ during SA (SL vs. SA,p<0.01) and to 200±6 ml·L⁻¹ at PP (SL vs. PP,p<0.05; SA vs. PP,p<0.01). Two-legged $\dot V_{O_{2max} } $ decreased from 3.64±0.26 L·min⁻¹ at SL to 2.70±0.14 L·min⁻¹ during SA (p<0.01) to 2.86±0.16 L·min⁻¹ at PP (p<0.01). One-legged $\dot V_{O_{2max} } $ decreased from 2.95±0.22 at SL to 2.25±0.17 L·min⁻¹ during SA (SL vs. SA,p<0.01) but improved to 2.66±0.18 L·min⁻¹ at PP (SA vs. PP,p<0.05). Since only one-legged $\dot V_{O_{2max} } $ increased as more oxygen was made available to the working muscles, the altitude-induced reduction in $\dot Q_{\max }$ can be implicated as being responsible for the reduction in $\dot V_{O_{2max} } $ during two-legged cycling.