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 (V̇O₂) 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 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 O₂ uptake was measured breath by breath and muscle oxygenation was assessed using near-infrared spectroscopy (NIRS). The NIRS 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 (τ) of the V̇O₂ response in phase 2 (bout 1: 29 ± 10 vs. bout 2: 28 ± 4 s; P = 0.91) but reduced the amplitude of the V̇O₂ slow component (bout 1: 0.45 ± 0.16 vs. bout 2: 0.22 ± 0.14 l/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 τ (bout 1: 38 ± 18 vs. bout 2: 24 ± 9 s; P = 0.03) but did not alter the amplitude of the V̇O₂ slow component (bout 1: 0.40 ± 0.29 vs. bout 2: 0.41 ± 0.20 l/min; P = 0.86). These results suggest that the performance of prior heavy exercise enables a speeding of phase 2 V̇O₂ 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 O ₂ supply was probably not limiting.