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UNLABELLED: Critical power (CP), respiratory compensation point (RCP), maximal lactate steady state (MLSS), and deoxyhemoglobin breakpoint (HHbBP) are alternative functional indices that are thought to demarcate the highest exercise intensity that can be tolerated for long durations. PURPOSE: We tested the hypothesis that CP, RCP, MLSS, and HHbBP occur at the same metabolic intensity by examining the pulmonary oxygen uptake (V˙)O2p and power output (PO) associated with each "threshold." METHODS: Twelve healthy men (mean ± SD age, 27 ± 3 yr) performed the following tests on a cycle ergometer: i) four to five exhaustive tests for determination of CP, ii) two to three 30-min constant-power trials for MLSS determination, and iii) a ramp incremental exercise test from which the V˙O2p and PO at RCP and HHbBP were determined. During each trial, breath-by-breath V˙O2p and ventilatory variables were measured with a metabolic cart and flowmeter turbine; near-infrared spectroscopy-derived HHb was monitored using a frequency domain multidistance system, and arterialized capillary blood lactate was sampled at regular intervals. RESULTS: There were no differences (P > 0.05) among the V˙O2p values associated with CP, RCP, MLSS, and HHbBP (CP, 3.29 ± 0.48; RCP, 3.34 ± 0.45; MLSS, 3.27 ± 0.44; HHbBP, 3.41 ± 0.46 L·min(-1)); however, the PO associated with RCP (262 ± 48 W) and HHbBP (273 ± 41 W) were greater (P 0.05). CONCLUSIONS: Although the standard methods for determination of CP, RCP, MLSS, and HHbBP are different, these indices occur at the same V˙O2p, suggesting that i) they may manifest as a result of similar physiological phenomenon and ii) each provides a valid delineation between tolerable and intolerable constant-power exercise.
Keir et al. (Wed,) studied this question.