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.