title: Exercise Intensity Thresholds: Identifying the Boundaries of Sustainable Performance.
abstract: 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.
  
  
  OBJECTIVE
  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 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 [HHb]BP 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 [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 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.
authors.affiliation: 1Canadian Centre for Activity and Aging, University of Western Ontario, London, Ontario, CANADA; 2School of Kinesiology, University of Western Ontario, London, Ontario, CANADA; 3Department of Neurological and Movement Sciences, University of Verona, Verona, ITALY; 4Faculty of Kinesiology, University of Calgary, Calgary, Alberta, CANADA; and 5Department of Physiology and Pharmacology, University of Western Ontario, London, Ontario, CANADA.
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Keir
authors.forename: Federico Y
authors.initials: FY
authors.lastname: Fontana
authors.forename: Taylor C
authors.initials: TC
authors.lastname: Robertson
authors.forename: Juan M
authors.initials: JM
authors.lastname: Murias
authors.forename: Donald H
authors.initials: DH
authors.lastname: Paterson
authors.forename: John M
authors.initials: JM
authors.lastname: Kowalchuk
authors.forename: Silvia
authors.initials: S
authors.lastname: Pogliaghi
mesh_terms: D000328:Adult; D015308:Anaerobic Threshold / Q000502:physiology; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D006454:Hemoglobins / Q000378:metabolism*; D006801:Humans; D019344:Lactic Acid / Q000097:blood*; D008297:Male; D010807:Physical Endurance / Q000502:physiology*; D015656:Respiratory Mechanics; D055815:Young Adult
chemical_list: D006454:Hemoglobins; D019344:Lactic Acid; C100003:deoxyhemoglobin