Effects of priming exercise on pulmonary O2 uptake and muscle deoxygenation kinetics during heavy-intensity cycle exercise in the supine and upright positions

We hypothesized that the performance of prior heavy exercise would speed the phase 2 oxygen consumption (Vo(2)) kinetics during subsequent heavy exercise in the supine position (where per-fusion pressure might limit muscle O-2 supply) but not in the upright position. Eight healthy men (mean +/- SD age 24 +/- 7 yr; body mass 75.0 +/- 5.8 kg) completed a double-step test protocol involving two bouts of 6 min of heavy cycle exercise, separated by a 10-min recovery period, on two occasions in each of the upright and supine positions. Pulmonary O-2 uptake was measured breath by breath and muscle oxygenation was assessed using near-infrared spectroscopy (NIRS). The MRS data indicated that the performance of prior exercise resulted in hyperemia in both body positions. In the upright position, prior exercise had no significant effect on the time constant (T) of the Vo(2) response in phase 2 (bout 1: 29 +/- 10 vs. bout 2: 28 +/- 4 s; P = 0.91) but reduced the amplitude of the Vo(2) slow component (bout 1: 0.45 +/- 0.16 vs. bout 2: 0.22 +/- 0. 14 1/min; P = 0.006) during subsequent heavy exercise. In contrast, in the supine position, prior exercise resulted in a significant reduction in the phase 2 T (bout 1: 38 +/- 18 vs. bout 2: 24 +/- 9 s; P = 0.03) but did not alter the amplitude of the Vo(2) slow component (bout 1: 0.40 +/- 0.29 vs. bout 2: 0.41 +/- 0.20 1/min; P = 0.86). These results suggest that the performance of prior heavy exercise enables a speeding of phase 2 Vo(2) kinetics during heavy exercise in the supine position, presumably by negating an O-2 delivery limitation that was extant in the control condition, but not during upright exercise, where muscle O-2 supply was probably not limiting.