The Respiratory Compensation “Point” as a Determinant of O2 Uptake Kinetics?
21. September 2012 (online)
In a recent issue of the International Journal of Sports Medicine, Pessoa Filho et al.  published a study characterising the O2 uptake ( V˙O2) kinetic responses to swimming exercise performed just below and above the respiratory compensation point (RCP). The investigators are to be commended for completing such an interesting and high-quality study. The investigators observed that swimming trials performed at velocities just below the RCP engendered a slowly developing component of increasing V˙O2 ( V˙O2sc), appearing to plateau at V˙O2 values equal to ~92% of peak O2 uptake by the end of the bout. This delayed ‘state-state’ response in V˙O2 is characteristic of exercise performed within the heavy-intensity domain (i. e., below the critical power; CP). Conversely, during swimming trials at velocities just above the RCP, the investigators reported that V˙O2 increased continuously, due to the V˙O2sc, toward values representing ~103% of peak O2 uptake – this V˙O2 kinetic response is typical of severe-intensity exercise (i. e., above CP)   . Based on these findings, the authors suggested that:
“…the RCP appears to represent a physiological boundary that dictates whether V˙O2 kinetics is characteristic of heavy- or severe-intensity exercise during swimming.”
Careful attention must be paid to the word “appears” in the foregoing extract, so that the findings of Pessoa Filho et al. are interpreted appropriately. In this study, the RCP does indeed appear to represent the CP – but looks can be deceiving! It is known that the RCP occurs at a lower power output during slow compared with fast ramping exercise protocols (i. e., 8 W · min − 1 vs. 65 W · min − 1) . If the incremental stage is extended to 4–5 min or longer, respiratory compensation for metabolic acidosis is observed at the first work rate performed above the gas-exchange threshold (GET)  , in which case: GET≈RCP. On the other hand, the weight of available evidence suggests that the GET and CP are distinct physiological thresholds that occur at 2 separate power outputs   .
The 300-m intervals used in this incremental swimming protocol  yielded an RCP that occurred, coincidentally, at a velocity similar to that expected for the CP  . Had the investigators used a longer incremental work stage (>300 m), a compensatory ventilatory response would have precipitated at a lower swimming velocity, and thus RCP<CP. Therefore, contrary to the authors’ opinions, the RCP does not represent a “physiological boundary” during constant-load exercise, demarcating the heavy/severe intensity domains. This point is clearly illustrated in a seminal paper by Poole et al.  who demonstrated that respiratory compensation, marked by a progressive decline in end-tidal partial pressure for CO2, was evident during exercise performed at the upper limit of the heavy-intensity domain (i. e., CP; see Fig. 4 in Poole et al. ). Moreover, Simon et al.  reported that compensatory hyperventilation was present during constant-load exercise performed at an intensity just below that corresponding to the RCP. Thus, respiratory compensation is likely to occur for all submaximal, constant workloads of sufficient duration, performed above the GET (i. e., heavy and severe exercise intensities).
We acknowledge that there are studies which use the RCP to define ‘training zones’ and the like. Indeed, our previous research   examining the relationships between the RCP, respiratory muscle work and the V˙O2sc amplitude also contributes to the misunderstanding that the RCP represents an important physiological ‘boundary’ or ‘threshold’ during constant-load exercise. However, when we let the evidence guide our convictions, we come to this conclusion: A discrete work rate defining the point of respiratory compensation does not appear to exist within the constant-load paradigm. One can only perform square-wave transitions at work rates (or velocities) equal to the RCP obtained during incremental exercise. Though this distinction may appear pedantic, we deem it necessary in order to avoid misconceptions about what is “physiological” and what is “coincidental”. Thus, while the RCP elicited during a 300-m incremental swimming protocol may coincide with the CP, one must be cautious in using such findings to suggest a mechanistic relationship between these 2 parameters.
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