NONLINEAR RELATIONSHIP BETWEEN O-2 UPTAKE AND POWER OUTPUT AT HIGH INTENSITIES OF EXERCISE IN HUMANS

1. A slow component to pulmonary oxygen uptake (V-O2) is reported during prolonged high power exercise performed at constant power output at, or above, approximately 60% of the maximal oxygen uptake. The magnitude of the slow component is reported to be associated with the intensity of exercise and to be largely accounted for by an increased V-O2 across the exercising legs. 2. On the assumption that the control mechanism responsible for the increased V-O2 is intensity dependent we hypothesized that it should also be apparent in multi-stage incremental exercise tests with the result that the V-O2-power output relationship would be curvilinear. 3. We further hypothesized that the change in the V-O2-power output relationship could be related to the hierarchical recruitment of different muscle fibre types with a lower mechanical efficiency. 4. Six subjects each performed five incremental exercise tests, at pedalling rates of 40, 60, 80, 100 and 120 rev min(-1), over which range we expected to vary the proportional contribution of different fibre types to the power output. Pulmonary V-O2 was determined continuously and arterialized capillary blood was sampled and analysed for blood lactate concentration ([lactate](b)). 5. Below the level at which a sustained increase in [lactate], was observed pulmonary V-O2 showed a linear relationship with power output; at high power outputs, however, there was an additional increase in V-O2 above that expected from the extrapolation of that linear relationship, leading to a positive curvilinear V-O2-power output relationship. 6. No systematic effect on the magnitude or onset of the 'extra' V-O2 was found in relation to pedalling rate, which suggests that it is not related to the pattern of motor unit recruitment in any simple way.