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

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Abstract
The effect of prior exercise on pulmonary O2 uptake (V̇o2p), 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 (Δ50%, 8 min; HVY 2) KE exercise. V̇o2p was measured breath-by-breath; LBF was measured by Doppler ultrasound at the femoral artery; and oxy (O2Hb)-, deoxy (HHb)-, and total (Hbtot) hemoglobin/myoglobin of the vastus lateralis muscle were measured continuously by near-infrared spectroscopy (NIRS; Hamamatsu NIRO-300). Phase 2 V̇o2p, 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 + τ) was calculated. Prior heavy-intensity exercise resulted in a speeding ( P < 0.05) of phase 2 V̇o2p 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 V̇o2p 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 τLBF [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 μM ( 10 ); HVY 2: 25 μM ( 10 ); P < 0.001] and the HHb-to-V̇o2p ratio [(HHb/V̇o2p) HVY 1: 14 μM·l−1·min−1 ( 6 ); HVY 2: 17 μM·l−1·min−1 ( 5 ); P < 0.05] were greater following prior heavy-intensity exercise. These results suggest that the speeding of phase 2 τV̇o2p was the result of both elevated local O2 availability and greater O2 extraction evidenced by the greater HHb amplitude and HHb/V̇o2p ratio following prior heavy-intensity exercise.

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