Response

Dear Editor-in-Chief, From our perspective, the respiratory compensation point (RCP) and near-infrared spectroscopy-derived muscle deoxygenation breakpoint ([HHb]BP) of ramp-incremental exercise represent two physiological manifestations that arise once a specific and equivalent metabolic rate is surpassed; both may be used interchangeably to identify this critical intensity (1–4). Boone and Bourgois (5) substantially agree that both RCP and [HHb]BP are triggered by “one single stimulus.” However, they also contend that compared with RCP, the [HHb]BP occurs at a higher oxygen uptake (V˙O2) and ramp-incremental power output (PO) (6,7). Their letter proposes that our practice of applying a time correction to determine the V˙O2 at [HHb]BP but not RCP may lead to an artifactual underestimation of the V˙O2 at [HHb]BP and a spurious RCP-[HHb]BP coincidence. The following is a clarification of our method and its rationale. Like Boone and Bourgois, we identify [HHb]BP from the [HHb] versus time relationship. Next, we apply a two-step process to determine first the V˙O2 at [HHb]BP and then its corresponding PO: Step 1: identify the V˙O2 at [HHb]BP. We left-shift V˙O2 relative to time by the mean response time (MRT) of V˙O2 to account for the muscle-to-mouth time delay and V˙O2 kinetics. Because there is no [HHb]–PO delay, the identification of the PO corresponding to the time at [HHb]BP (at this point) would not require a correction. Step 2: estimate the constant PO that would be expected to elicit the V˙O2 at [HHb]BP. We left-shift V˙O2 relative to ramp PO by the PO equivalent of MRT (ramp rate [W·s−1] × MRT [s]) and then identify the PO corresponding to the V˙O2 at [HHb]BP. Conversely, the same method could be applied to identify the V˙O2 corresponding to the PO identified at [HHb]BP. For the determination of the V˙O2 at RCP (i.e., step 1), no shifting is required. The RCP is identified by plotting ventilation and end-tidal CO2 tension against V˙O2. As such, V˙O2 need not be shifted because metabolic-induced ventilatory changes (response) occur at the same time or shortly after the muscle V˙O2 that initiated them (stimulus) is expressed systemically and at the mouth. However, when estimating the constant PO (i.e., step 2) expected to elicit the V˙O2 at either [HHb]BP or RCP, data shifting must be applied. With step 2, we acknowledge that correction based on MRT only ignores the V˙O2 slow component and therefore will overestimate the PO at a given V˙O2 or underestimate the V˙O2 at a given PO (8,9). However, these errors are nullified if [HHb]BP and RCP are measured and compared in terms of V˙O2 rather than a translated PO equivalent (8). Caen et al. (7) are the first to describe a difference in PO at [HHb]BP versus RCP (~20 W). These findings are unprecedented (1–4) and even contradict previous work from the same group (6) reporting a strong correlation (r = 0.96) and no difference (0.8% ± 1.3% V˙O2peak; equivalent to <4 W) between [HHb]BP and RCP. Curiously, the 20-W difference (7) is approximately what one would expect if MRT-corrected POs (i.e., RCP) are compared with uncorrected ones (i.e., [HHb]BP) (e.g., 0.5 W·s−1 ramp × 40 s MRT). Available evidence indicates that [HHb]BP and RCP are equivalent.

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