Skeletal Muscle Oxygen Uptake Dynamics during the Exercise Off‐Transient

The time course of muscle V̇O 2 recovery (i.e., muscle V̇O 2 kinetics) from contractions, measured at the site of O 2 exchange i.e., in the microcirculation, is unknown. It is known that, in the rat spinotrapezius muscle, blood flow (Qm) kinetics are slower in recovery (time constant, t ~41 s) than at the onset of contractions (t ~17 s, Ferreira et al. J. Physiol. 2006). Purpose We tested the hypothesis that this Qm kinetics profile was associated with a slowed m V̇O 2 recovery compared with that seen at the onset of contractions (t ~23 s, Behnke et al. Resp. Physiol. 2002). Methods Measurements of capillary red blood cell flux and microvascular PO 2 (PO 2 ) were combined to resolve the time course of muscle V̇O 2 across the moderate intensity contractions‐to‐rest transition. Results Contrary to our hypothesis, in recovery muscle V̇O 2 began to decrease immediately (i.e., time delay 0 s in all but one animal) and demonstrated rapid first order‐kinetics (t 22.4 ± 2.1 s). Conclusions The slowed Qm kinetics in recovery serves to elevate the Qm/ V̇O 2 ratio and thus microvascular PO 2 . Whether this Qm response is obligatory to the rapid muscle V̇O 2 kinetics and hence speeds the repletion of high‐energy phosphates by maximizing conductive and diffusive O 2 flux is an important question that awaits resolution. Support NIH AG031327.