The purpose of this study was to determine the relationship between oxygen uptake (<(V)over dot>O(2)) and the oxygen supply system during incremental load in the supine position. Each subject (n=6) performed an incremental-load supine exercise test (20 W min(-1)). <(V)over dot>O(2) and heart rate (HR) were measured breath-by-breath during exercise. Oxygenation level was obtained from the inactive biceps brachii muscle using near-infrared spectroscopy (NIRS).<(V)over dot>O(2) and HR increased linearly in proportion to increase in work rate (WR) after a time delay. The linear relationships between <(V)over dot>O(2) and WR and between HR and WR below the ventilatory threshold (VT, 112.9 +/- 11.7 W) were extrapolated to WR above the VT. Estimated HR was not significantly different from that measured above VT. Oxygenation level slightly decreased after the VT and decreased exponentially after the respiratory compensation point (RCP, 175.8 +/- 21.3 W). Oxygenation level decreased from 100% at warming-up exercise (10 W) to 63.3 +/- 14.0% at exhaustion. In this study, HR and oxygenation level were regarded as oxygen supply to the whole body in the supine position and oxygen supply to inactive muscle, respectively. Therefore, an increase in FIR suggests a linear increase in oxygen supply to active muscle through exercise. In addition to the effect of HR, a decrease in oxygenation level in inactive muscle could induce an increase in oxygen supply to active muscle. However, <(V)over dot>O(2) showed a linear increase throughout the exercise in the supine position. Judging from the present indirect indicators of FIR and oxygenation level, it is unlikely that <(V)over dot>O(2) kinetics is determined by oxygen supply to active muscle in the supine position.