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1 Sport and Health Sciences, Exeter University, EXETER, United Kingdom; , United Kingdom
2 Sport and Health Sciences, Exeter University, EXETER, United Kingdom
3 School of Medicine, Exeter University, United Kingdom
4 Department of Kinesiology & Anatomy & Physiology, Kansas State University, Manhattan, Kansas, United States
* To whom correspondence should be addressed. E-mail: a.m.jones{at}exeter.ac.uk.
We tested the hypothesis that the asymptote of the hyperbolic relationship between work-rate and the time-to-exhaustion during muscular exercise, the so-called critical power (CP), represents the highest constant-work-rate that can be sustained without a progressive loss of homeostasis (as assessed using 31P-MRS measurements of muscle metabolites). Six healthy male subjects initially completed single-leg knee-extension exercise at 3-4 different constant-work-rates to the limit of tolerance for estimation of the CP (mean±SD, 20±2 W). Subsequently, the subjects exercised at work-rates that were 10% below CP (for 20 min) and 10% above CP (for as long as possible) while the metabolic responses in the contracting quadriceps muscle ([PCr], [Pi] and pH) were estimated using 31P-MRS. All subjects completed 20 min of exercise <CP without duress, whereas the limit of tolerance during >CP exercise was 14.7±7.1 min. During exercise <CP, stable values for [PCr], [Pi] and pH were attained within 3 minutes following the onset of exercise and there were no further significant changes in these variables (end-exercise [PCr]:67±11% of baseline; end-exercise [Pi]:314±216% of baseline; end-exercise pH:7.01±0.03). During exercise >CP, however, [PCr] continued to fall to the point of exhaustion and [Pi] and pH changed precipitously (end-exercise [PCr]:27±17% of baseline; end-exercise [Pi]:564±167% of baseline; end-exercise pH:6.87±0.10; all P<0.05 compared to <CP exercise). These data support the hypothesis that the CP represents the highest constant-work-rate that can be sustained without a progressive depletion of muscle high-energy phosphates and a rapid accumulation of metabolites (i.e. [H+], [Pi]) that have been associated with the fatigue process.
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