Determinants of oxygen consumption during exercise on cycle ergometer: The effects of gravity acceleration

The hypothesis that changes in gravity acceleration (ag) affect the linear relationships between oxygen consumption (V˙O2) and mechanical power (w˙) so that at any w˙, V˙O2 increases linearly with ag was tested under conditions where the weight of constant-mass legs was let to vary by inducing changes in ag in a human centrifuge. The effects of ag on the V˙O2/w˙ relationship were studied on 14 subjects at two pedalling frequencies (fp, 1.0 and 1.5 Hz), during four work loads on a cycle ergometer (25, 50, 75 and 100 W) and at four ag levels (1.0, 1.5, 2.0 and 2.5 times normal gravity). V˙O2 increased linearly with w˙. The slope did not differ significantly at various ag and fp, suggesting invariant mechanical efficiency during cycling, independent of fp and ag. Conversely, the y-intercept of the V˙O2/w˙ relationship, defined as constant b, increased linearly with ag. Constant b is the sum of resting V˙O2 plus internal metabolic power (E˙i). Since the former was the same at all investigated ag, the increase in constant b was entirely due to an increase in E˙i. Since the V˙O2 versus w˙ lines had similar slopes, the changes in E˙i entirely explained the higher V˙O2 at each w˙, as ag was increased. In conclusion, the effects of ag on V˙O2 are mediated through changes in E˙i, and not in w˙ or in resting V˙O2.