Exercise intensity thresholds: identifying the boundaries of sustainable performance.

Critical power (CP), respiratory compensation point (RCP), maximal lactate steady-state (MLSS), and deoxyhemoglobin ([HHb]) 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˙O2p) as well as power output (PO) associated with each "threshold".METHODS: Twelve healthy men (mean±SD age: 27±3 years) performed the following tests on a cycle ergometer: i) four to five exhaustive tests for determination of CP; ii) two to three, 30-minute constant-power trials for MLSS determination; and iii) a ramp incremental exercise test from which the V˙O2p and PO at RCP and [HHb] BP were determined. During each trial, breath-by-breath V˙O2p and ventilatory variables were measured with a metabolic cart and flow-meter turbine; near-infrared spectroscopy-derived [HHb] was monitored using a frequency domain multi-distance system, and arterialized-capillary blood lactate was sampled at regular intervals.RESULTS: There were no differences (p>0.05) amongst the V˙O2p 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); 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 V˙O2p suggesting that: i) they may manifest as a result of similar physiological phenomenon; ii) each provides a valid delineation between tolerable and intolerable constant-power exercise. CONCLUSIONS: Although the standard methods for determination of CP, RCP, MLSS, and [HHb]BP 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.