Kinetics of VO2 limb blood flow and regional muscle deoxygenation in young adults during moderate intensity, knee-extension exercise.

The kinetics of pulmonary O(2) uptake (VO(2p)), limb blood flow (LBF) and deoxygenation (DeltaHHb) of the vastus lateralis (VL) and vastus medialis (VM) muscles during the transition to moderate-intensity knee-extension exercise (MOD) was examined. Seven males (27 +/- 5 years; mean +/- SD) performed repeated step transitions (n = 4) from passive exercise to MOD. Breath by breath VO(2p) femoral artery LBF, and VL and VM muscle DeltaHHb were measured, respectively, by mass spectrometer and volume turbine, Doppler ultrasound and near-infrared spectroscopy. Phase 2 VO(2p), LBF, and HHb data were fit with a mono-exponential model. The time constant (tau) of the VO(2p) and LBF response were not different (tauVO(2p), 24 +/- 6 s; tauLBF, 23 +/- 8 s). The DeltaHHb response did not differ between VL and VM in amplitude (VL 6.97 +/- 4.22 a.u.; VM 7.24 +/- 3.99 a.u.), time delay (DeltaHHb(TD): VL 17 +/- 2 s; VM 15 +/- 1 s), time constant (tauDeltaHHb: VL 11 +/- 6 s; VM 13 +/- 4 s), or effective time constant [tau'DeltaHHb (= DeltaHHb(TD) + tauDeltaHHb): VL 28 +/- 7 s; VM 28 +/- 4 s]. Adjustments in DeltaHHb in VL and VM depict a similar balance of regional O(2) delivery and utilization within the quadriceps muscle group. The tau'DeltaHHb and tauVO(2p) were similar, however, the DeltaHHb displayed an "overshoot" relative to the steady-state levels reflecting a slower alteration of microvascular blood flow (O(2) delivery) relative to O(2) utilization, necessitating a greater reliance on O(2) extraction.