Response

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Dear Editor-in-Chief,
We thank Boone and Bourgois (1) for their interest in our recent article (4). Their comments are interesting and add value to the ongoing debate on whether the respiratory compensation point (RCP) is a valid surrogate for the critical power (CP). We agree that the estimation and subsequent adjustment of the mean response time (MRT), to account for the delay in the pulmonary O2 uptake kinetic response inherent to incremental exercise, may, to some extent, explain the poor measurement agreement commonly observed between the RCP and the CP. The relatively poor reproducibility and protocol dependency of the MRT (2), coupled with the apparent nonlinear MRT dynamics for external work rates incurred above the gas-exchange threshold (GET) (3), are factors that may affect the accuracy/confidence of the MRT estimate obtained during incremental exercise. Whether such observations are physiological and/or methodological in nature is unclear. Notwithstanding the issues associated with estimating the MRT during incremental exercise, there is a wealth of experimental evidence supporting the idea that, for supra-GET external work rates, respiratory compensation manifests in a time-dependent manner (4). In fact, respiratory compensation may be observed at an external work rate corresponding to the GET during slow incremental protocols (5) and prolonged constant-load exercise (6). In contrast, the RCP is not discernable in most individuals during fast ramp-incremental protocols (i.e., 65 W·min−1) (7). Thus, the time-dependent nature of respiratory compensation, rather than the MRT, may explain most of the disagreement between the RCP and the CP.
Finally, we do not agree that a longitudinal exercise training intervention is the only experimental approach to determine whether the RCP and the CP are mechanistically “linked.” Other interventions that manipulate O2 provision and/or O2 utilization, such as hypoxia and hyperoxia, may also yield further insights into the relationship between the RCP and the CP. Nevertheless, it is of interest that Wang et al. (8) recently investigated the correspondence between the RCP and the CP (determined from the 3-min “all-out” cycling test) before and after a 4-wk high-intensity interval training program. In that study, the investigators reported poor absolute measurement agreement between the RCP and the CP at both measurement times, as evidenced by wide limits of agreement (Bland–Altman plots; see Fig. 3 in Wang et al. [8]), despite occurring, on average, at similar external work rates. Furthermore, although the external work rate associated with both physiological indices significantly increased after training, the delta change in the RCP did not mirror that of the CP (see Fig. 4 in Wang et al. [8]). Therefore, it appears that the RCP and the CP do not respond to exercise training with a similar time course, further supporting the notion that these two physiological “thresholds,” when expressed as an external work rate, are not synonymous.
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