Effect of priming exercise and body position on pulmonary oxygen uptake and muscle deoxygenation kinetics during cycle exercise.

We hypothesized that the performance of prior heavy exercise would speed pulmonary oxygen uptake ((V) over dot(O2)) kinetics (i.e., as described by the time constant, tau(V) over dot(O2)) and reduce the amplitude of muscle deoxygenation (deoxy[heme]) kinetics in the supine (S) but not upright (U) body position. Seventeen healthy men completed heavy-intensity constantwork rate exercise tests in S and U consisting of two bouts of 6-min cycling separated by 6-min cycling at 20 W. Pulmonary (V) over dot(O2) was measured breath by breath; total and deoxy[heme] were determined via time-resolved near-infrared spectroscopy (NIRS) at three muscle sites. Priming exercise reduced (V) over dot(O2) in S (bout 1: 36 +/- 10 vs. bout 2: 28 +/- 10 s, P < 0.05) but not U (bout 1: 27 +/- 8 s vs. bout 2: 25 +/- 7 s, P > 0.05). Deoxy[heme] amplitude was increased after priming in S (bout 1: 25-28 mu M vs. bout 2: 30-35 mu M, P < 0.05) and U (bout 1: 13-18 mu M vs. bout 2: 17-25 mu M, P > 0.05), whereas baseline total[heme] was enhanced in S (bout 1: 110-179 mu M vs. bout 2: 121-193 mu M, P < 0.05) and U (bout 1: 123-186 mu M vs. bout 2: 137-197 mu M, P < 0.05). Priming exercise increased total[heme] in both S and U, likely indicating enhanced diffusive O-2 delivery. However, the observation that after priming the amplitude of the deoxy[heme] response was increased in S suggests that the reduction in tau(V) over dot(O2)) subsequent to priming was related to a combination of both enhanced intracellular O-2 utilization and increased O-2 delivery. NEW & NOTEWORTHY Here we show that oxygen uptake ((V) over dot(O2)) kinetics are slower in the supine compared with upright body position, an effect that is associated with an increased amplitude of skeletal muscle deoxygenation in the supine position. After priming in the supine position, the amplitude of muscle deoxygenation remained markedly elevated above that observed during upright exercise. Hence, the priming effect cannot be solely attributed to enhanced O-2 delivery, and enhancements to intracellular O-2 utilization must also be contributory.