Exertional oxygen uptake kinetics: a stamen of stamina?

Abstract

The fundamental pulmonary O(2) uptake (.VO(2)) response to moderate, constant-load exercise can be characterized as (d.VO(2)/dt)(tau)+Delta.VO(2) (t)=Delta.VO(2SS) where Delta.VO(2SS) is the steady-state response, and tau is the time constant, with the .VO(2) kinetics reflecting intramuscular O(2) uptake (.QO(2)) kinetics, to within 10%. The role of phosphocreatine (PCr) turnover in .QO(2) control can be explored using (31)P-MR spectroscopy, simultaneously with .VO(2). Although tau.VO(2) and tauPCr vary widely among subjects (approx. 20-65 s), they are not significantly different from each other, either at the on- or off-transient. A caveat to interpreting the "well-fit" exponential is that numerous units of similar Delta.VO(2SS) but with a wide tau distribution can also yield a .VO(2) response with an apparent single tau. This tau is, significantly, inversely correlated with lactate threshold and .VO(2max)(but is poorly predictive; a frail stamen, therefore), consistent with tau not characterizing a compartment with uniform kinetics. At higher intensities, the fundamental kinetics become supplemented with a slowly-developing phase, setting .VO(2)on a trajectory towards maximum .VO(2). This slow component is also demonstrable in Delta[PCr]: the decreased efficiency thereby reflecting a predominantly high phosphate-cost of force production rather than a high O(2)-cost of phosphate production. We also propose that the O(2)-deficit for the slow-component is more likely to reflect shifting Delta.VO(2SS) rather than a single one with a single tau.

The slow component of V? O 2 kinetics in very heavy and fatiguing square-wave exercise

“…Therefore, the as yet undetermined mechanisms underlying the slow component may recur in stepwise fashion throughout prolonged exercise at high intensities. However, our results can not fully support the hypothesis that the entire slow component is comprised of many small increments (Whipp et al 2002); studies relying on pulmonary oxygen consumption measurements are probably not sensitive enough to uncover such subtle intramuscular kinetics if they exist. If each of the TD points found in this study represents a critical period of development or increased magnitude of the mechanisms underlying the slow component, then longer bout lengths at exercise intensities eliciting multiple increments should provide useful mechanistic insight in future studies.…”

“…It has been proposed that many small increments with rapid time constants overlap and give the appearance of a smooth increment response, as traditionally modeled with a single slow component increment, when measured from expired gases at the mouth (Whipp et al 2002). Our data provide partial evidence for this.…”

“…It is difficult to hypothesize a mechanism that could account for such a large and sustained increase in _ V O 2 beginning at a single time delay some minutes into the bout. It is more plausible that the slow component is composed of progressive increases in _ V O 2 rather than a single and abrupt increase in oxygen demand (Whipp et al 2002). Such a finding would be more consistent with dynamic maintenance of energy balance than a single large error signal implied by a mono-exponential slow component.…”

Determinants of performance in 1,500-m runners

“…Data obtained during the first 20 s of the transition (corresponding to the ''cardiodynamic phase'') were excluded from the analysis . Thus, the analysis of _ VO 2 kinetics mainly dealt with the ''phase 2'' (or ''primary'' component) of the response, which should closely reflect gas exchange kinetics at the skeletal muscle level (Grassi et al 1996;Whipp et al 2002). To mathematically evaluate the _ VO 2 on-kinetics, and to check for the presence of a _ VO 2 ''slow component'' (Jones et al 2011), data were fitted by a function of the following type:…”

Effects of ammonium chloride ingestion on phosphocreatine metabolism during moderate- and heavy-intensity plantar-flexion exercise

“…Further, the adequacy of the [PCr] kinetic analysis was shown by the time constants being in the expected range (*20-65 s) (Whipp et al 2002), the high correlations between the data and the exponential model line of best fit (R = 0.90-0.99), and visual inspection of the residuals, which fell evenly on either side of the ''zero difference line''. for a representative subject in Fig.…”