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