The effect of increasing work rate amplitudes from a common metabolic baseline on the kinetic response of V̇O2p, blood flow, and muscle deoxygenation.

Abstract

A step-transition in external work rate (WR) increases pulmonary O2 uptake (V̇O2p) in a monoexponential fashion. While 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 O2 delivery to the muscle (via blood flow; BF), a potential regulator of V̇O2p kinetics, has not been extensively studied. To investigate the role of BF on V̇O2p kinetics, ten 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 4-6 times. Volume turbinometry and mass spectrometry, Doppler ultrasound, and near-infrared spectroscopy were used to measure V̇O2p, BF, and muscle deoxygenation (deoxy[Hb+Mb]), respectively. Like transitions were ensemble-averaged and phase II V̇O2p, BF, and deoxy[Hb+Mb] were fit with a monoexponential non-linear least squares regression equation. With increasing ΔWR, τV̇O2p became larger at the higher ΔWRs (p < 0.05), while τBF did not change significantly, and the mean response time (MRT) of deoxy[Hb+Mb] became smaller. These findings that V̇O2p kinetics become slower with increasing ΔWR, while BF kinetics are not influenced by ΔWR, suggest that O2 delivery could not limit V̇O2p in this situation. However, the speeding of deoxy[Hb+Mb] kinetics with increasing ΔWR does imply that the O2 delivery-to-O2 utilization of the microvasculature decreases at higher ΔWRs. This suggests that the contribution of O2 delivery and O2 extraction to V̇O2 in the muscle change with increasing ΔWR.