We manipulated the baseline metabolic rate and body position to explore the effect of the interaction between recruitment of discrete sections of the muscle fiber pool and muscle O(2) delivery on pulmonary O(2) uptake (VO(2)) kinetics during cycle exercise. We hypothesized that phase II VO(2) kinetics (tau(p)) in the transition from moderate- to severe-intensity exercise would be significantly slower in the supine than upright position because of a compromise to muscle perfusion and that a priming bout of severe-intensity exercise would return tau(p) during supine exercise to tau(p) during upright exercise. Eight male subjects [35 +/- 13 (SD) yr] completed a series of "step" transitions to severe-intensity cycle exercise from an "unloaded" (20-W) baseline and a baseline of moderate-intensity exercise in the supine and upright body positions. tau(p) was not significantly different between supine and upright exercise during transitions from a 20-W baseline to moderate- or severe-intensity exercise but was significantly greater during moderate- to severe-intensity exercise in the supine position (54 +/- 19 vs. 38 +/- 10 s, P < 0.05). Priming significantly reduced tau(p) during moderate- to severe-intensity supine exercise (34 +/- 9 s), returning it to a value that was not significantly different from tau(p) in the upright position. This effect occurred in the absence of changes in estimated muscle fractional O(2) extraction (from the near-infrared spectroscopy-derived deoxygenated Hb concentration signal), such that the priming-induced facilitation of muscle blood flow matched increased O(2) utilization in the recruited fibers, resulting in a speeding of VO(2) kinetics. These findings suggest that, during supine cycling, priming speeds VO(2) kinetics by providing an increased driving pressure for O(2) diffusion in the higher-order (i.e., type II) fibers, which would be recruited in the transition from moderate- to severe-intensity exercise and are known to be especially sensitive to limitations in O(2) supply.