Exercise Intensity Thresholds: Identifying the Boundaries of Sustainable Performance

Critical power (CP), respiratory compensation point (RCP), maximal lactate steady state (MLSS), and deoxyhemoglobin breakpoint ([HHb]^BP) 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 [HHb]^BP occur at the same metabolic intensity by examining the pulmonary oxygen uptake (V)O^2p 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 VO^2p and PO at RCP and [HHb]^BP were determined. During each trial, breath-by-breath VO^2p 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 VO^2p values associated with CP, RCP, MLSS, and [HHb]^BP (CP, 3.29 ± 0.48; RCP, 3.34 ± 0.45; MLSS, 3.27 ± 0.44; [HHb]^BP, 3.41 ± 0.46 L·min^-1); however, the PO associated with RCP (262 ± 48 W) and [HHb]^BP (273 ± 41 W) were greater (P < 0.05) than both CP (226 ± 45 W) and MLSS (223 ± 39 W), which, themselves, were not different (P > 0.05). Conclusions Although the standard methods for determination of CP, RCP, MLSS, and [HHb]^BP are different, these indices occur at the same VO^2p, 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.