Oxygen cost of dynamic leg exercise on a cycle ergometer: effects of gravity acceleration

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Abstract
A model of the metabolic internal power (Ėint) during cycling, which includes the gravity acceleration (ag) as a variable, is presented. This model predicts that Ėint is minimal in microgravity (0 g; g=9.81 m s–2), and increases linearly with ag, whence the hypothesis that the oxygen uptake (V˙O2) during cycling depends on ag. Repeated V˙O2 measurements during steady-state exercise at 50, 75 and 100 W on the cycle ergometer, performed in space (0 g) and on Earth (1 g) on two subjects, validated the model. V˙O2 was determined from the time course of decreasing O2 fraction during rebreathing. The gas volume during rebreathing was determined by the dilution principle, using an insoluble inert gas (SF6). Average V˙O2 for subject 1 at each power was 0.99, 1.21 and 1.52 L min–1 at 1 g (n=3) and 0.91, 1.13 and 1.32 L min–1 at 0 g (n=5). For subject 2 it was 0.90, 1.12 and 1.42 L min–1 at 1 g, and 0.76, 0.98 and 1.21 L min–1 at 0 g. These values corresponded to those predicted from the model. Although resting V˙O2 was lower at 0 g than at 1 g, the net (total minus resting) exercise V˙O2 was still smaller at 0 g than at 1 g. This difference reflects the lower Ėint at 0 g.