The effect of increasing work rate amplitudes from a common metabolic baseline on the kinetic response of V̇o, blood flow, and muscle deoxygenation.

A step-transition in external work rate (WR) increases pulmonary O uptake (V̇o) in a monoexponential fashion. Although the rate of this increase, quantified by the time constant (τ), has frequently been shown to be similar between multiple different WR amplitudes (ΔWR), the adjustment of O delivery to the muscle (via blood flow; BF), a potential regulator of V̇o kinetics, has not been extensively studied. To investigate the role of BF on V̇o kinetics, 10 participants performed step-transitions on a knee-extension ergometer from a common baseline WR (3 W) to: 24, 33, 45, 54, and 66 W. Each transition lasted 8 min and was repeated four to six times. Volume turbinometry and mass spectrometry, Doppler ultrasound, and near-infrared spectroscopy were used to measure V̇o, BF, and muscle deoxygenation (deoxy[Hb + Mb]), respectively. Similar transitions were ensemble-averaged, and V̇o, BF, and deoxy[Hb + Mb] were fit with a monoexponential nonlinear least squares regression equation. With increasing ΔWR, τV̇o became larger at the higher ΔWRs ( < 0.05), while τBF did not change significantly, and the mean response time (MRT) of deoxy[Hb + Mb] became smaller. These findings that V̇o kinetics become slower with increasing ΔWR, while BF kinetics are not influenced by ΔWR, suggest that O delivery could not limit V̇o in this situation. However, the speeding of deoxy[Hb + Mb] kinetics with increasing ΔWR does imply that the O delivery-to-O utilization of the microvasculature decreases at higher ΔWRs. This suggests that the contribution of O delivery and O extraction to V̇O in the muscle changes with increasing ΔWR. A step increase in work rate produces a monoexponential increase in V̇o and blood flow to a new steady-state. We found that step transitions from a common metabolic baseline to increasing work rate amplitudes produced a slowing of V̇o kinetics, no change in blood flow kinetics, and a speeding of muscle deoxygenation kinetics. As work rate amplitude increased, the ratio of blood flow to V̇o became smaller, while the amplitude of muscle deoxygenation became greater. The gain in vascular conductance became smaller, while kinetics tended to become slower at higher work rate amplitudes.