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 ((V) over dotO(2p)); limb blood flow (LBF) and deoxygenation (Delta HHb) 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 (V) over dotO(2p); femoral artery LBF, and VL and VM muscle DHHb were measured, respectively, by mass spectrometer and volume turbine, Doppler ultrasound and near-infrared spectroscopy. Phase 2 (V) over dotO(2p); LBF, and DHHb data were fit with a mono-exponential model. The time constant (tau) of the (V) over dotO(2p) and LBF response were not different (tau(V) over dotO(2p); 24 +/- 6 s; tau LBF, 23 +/- 8 s). The Delta HHb 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 (Delta HHb(TD): VL 17 +/- 2 s; VM 15 +/- 1 s), time constant (tau Delta HHb: VL 11 +/- 6 s; VM 13 +/- 4 s), or effective time constant [tau'Delta HHb (= Delta HHb(TD) + tau Delta HHb): VL 28 +/- 7 s; VM 28 +/- 4 s]. Adjustments in Delta HHb in VL and VM depict a similar balance of regional O-2 delivery and utilization within the quadriceps muscle group. The tau'Delta HHb and tau(V) over dotO(2p) were similar, however, the DHHb 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.