Recovery dynamics of skeletal muscle oxygen uptake during the exercise off-transient

The time course of muscle V˙O2 recovery from contractions (i.e., muscle V˙O2 off-kinetics), measured directly at the site of O2 exchange, i.e., in the microcirculation, is unknown. Whereas biochemical models based upon creatine kinase flux rates predict slower V˙O2 off- than on-transients [Kushmerick, M.J., 1998. Comp. Biochem. Physiol. B: Biochem. Mol. Biol.] whole muscle V˙O2 data [Krustrup, et al. J. Physiol.] suggest on–off symmetry.PurposeWe tested the hypothesis that the slowed recovery blood flow (Qm) kinetics profile in the spinotrapezius muscle [Ferreira et al., 2006. J. Physiol.] was associated with a slowed muscle V˙O2 recovery compared with that seen at the onset of contractions (time constant, τ ∼ 23 s, Behnke et al., 2002. Resp. Physiol.), i.e., on–off asymmetry.MethodsMeasurements of capillary red blood cell flux and microvascular pressure of O2(PO2mv)(PO2mv) were combined to resolve the temporal profile of muscle V˙O2 across the moderate intensity contractions-to-rest transition.ResultsMuscle V˙O2 decreased from an end-contracting value of 7.7 ± 0.2 ml/100 g/min to 1.7 ± 0.1 ml/100 g/min at the end of the 3 min recovery period, which was not different from pre-stimulation V˙O2. Contrary to our hypothesis, muscle V˙O2 in recovery began to decrease immediately (i.e., time delay <2 s) and demonstrated rapid first-order kinetics (τ, 25.5 ± 2.6 s) not different (i.e., symmetrical to) to those during the on-transient. This resulted in a systematic increase in microvascular PO2PO2 during the recovery from contractions.ConclusionsThe slowed Qm kinetics in recovery serves to elevate the Qm/V˙O2 ratio and thus microvascular PO2PO2. Whether this Qm response is obligatory to the rapid muscle V˙O2 kinetics and hence speeds the repletion of high-energy phosphates by maximizing conductive and diffusive O2 flux is an important question that awaits resolution.