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

A model of the metabolic internal power (Ė(int)) during cycling, which includes the gravity acceleration (a(g)) as a variable, is presented. This model predicts that Ė(int) is minimal in microgravity (0 g; g = 9.81 m s-2), and increases linearly with a(g), whence the hypothesis that the oxygen uptake (V̇O2) during cycling depends on a(g). Repeated V̇O2 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. V̇O2 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 V̇O2 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 V̇O2 was lower at 0 g than at 1 g, the net (total minus resting) exercise V̇O2 was still smaller at 0 g than at 1 g. This difference reflects the lower Ė(int) at 0 g.