Prior exercise speeds pulmonary O2 uptake kinetics by increases in both local muscle O2 availability and O2 utilization

The effect of prior exercise on pulmonary O-2 uptake (Vo(2p)), leg blood flow (LBF), and muscle deoxygenation at the onset of heavy-intensity alternate-leg knee-extension (KE) exercise was examined. Seven subjects [27 (5) yr; mean (SD)] performed step transitions (n = 3; 8 min) from passive KE following no warm-up (HVY 1) and heavy-intensity (Delta 50%, 8 min; HVY 2) KE exercise. Vo(2p) was measured breath-by-breath; LBF was measured by Doppler ultrasound at the femoral artery; and oxy (O(2)Hb)-, deoxy (HHb)-, and total (Hb(tot)) hemoglobin/myoglobin of the vastus lateralis muscle were measured continuously by near-infrared spectroscopy (NIRS; Hamamatsu NIRO-300). Phase 2Vo(2p), LBF, and HHb data were fit with a monoexponential model. The time delay (TD) from exercise onset to an increase in HHb was also determined and an HHb effective time constant (HHb - MRT = TD + tau) was calculated. Prior heavy-intensity exercise resulted in a speeding (P < 0.05) of phase 2Vo(2p) kinetics [HVY 1: 42 s (6); HVY 2: 37 s (8)], with no change in the phase 2 amplitude [HVY 1: 1.43 l/min (0.21); HVY 2: 1.48 l/min (0.21)] or amplitude of the Vo(2p) slow component [HVY 1: 0.18 l/min (0.08); HVY 2: 0.18 l/min (0.09)]. O2Hb and Hbtot were elevated throughout the on-transient following prior heavy-intensity exercise. The tau LBF [HVY 1: 39 s (7); HVY 2: 47 s (21); P = 0.48] and HHb-MRT [HVY 1: 23 s (4); HVY 2: 21 s (7); P = 0.63] were unaffected by prior exercise. However, the increase in HHb [HVY 1: 21 mu M (10); HVY 2: 25 mu M (10); P < 0.001] and the HHb-to-Vo(2p) ratio [(HHb/Vo(2p)) HVY 1: 14 mu M center dot l(-1) center dot min(-1) (6); HVY 2: 17 mu M center dot l(-1) center dot min(-1) (5); P < 0.05] were greater following prior heavy-intensity exercise. These results suggest that the speeding of phase 2 tau Vo(2p) was the result of both elevated local O-2 availability and greater O-2 extraction evidenced by the greater HHb amplitude and HHb/Vo(2p) ratio following prior heavy-intensity exercise.