We hypothesized that the performance of prior heavy exercise would speed the Phase II kinetics during subsequent heavy exercise in the supine position (where perfusion pressure might limit muscle O₂ supply) but not in the upright position. Eight healthy males (mean ± S.D. age 24 ± 7 years; body mass 75.0 ± 5.8 kg) completed a double-step test protocol involving 2 bouts of 6 minutes of heavy cycle exercise, separated by a 10 minute recovery period, on two occasions in each of the upright and supine positions. Pulmonary O₂ uptake was measured breath-by-breath and muscle oxygenation was assessed using near infra-red spectroscopy (NIRS). The NIRS data indicated that the performance of prior exercise resulted in a hyperemia in both body positions. In the upright position, prior exercise had no significant effect on the time constant () of the response in Phase II (Bout 1: 29 ± 10 vs. Bout 2: 28 ± 4 s; P=0.91) but reduced the amplitude of the slow component (Bout 1: 0.45 ± 0.16 vs. Bout 2: 0.22 ± 0.14 L^.min^-1; P=0.006) during subsequent heavy exercise. In contrast, in the supine position, prior exercise resulted in a significant reduction in the Phase II (Bout 1: 38 ± 18 vs. Bout 2: 24 ± 9 s; P=0.03) but did not alter the amplitude of the slow component (Bout 1: 0.40 ± 0.29 vs. Bout 2: 0.41 ± 0.20 L^.min^-1; P=0.86). These results suggest that the performance of prior heavy exercise enables a speeding of Phase II kinetics during heavy exercise in the supine position, presumably by negating an O₂ delivery limitation that was extant in the control condition, but not during upright exercise, where muscle O₂ supply was probably not limiting.