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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Muscle metabolic responses to exercise above and below the "critical power" assessed using ³¹P-MRS

¹School of Sport and Health Sciences and ²Peninsula School of Medicine, St. Luke's Campus, University of Exeter, Exeter, Devon, United Kingdom; and ³Departments of Kinesiology and Anatomy and Physiology, Kansas State University, Manhattan, Kansas

Submitted 9 October 2007 ; accepted in final form 1 December 2007

We tested the hypothesis that the asymptote of the hyperbolic relationship between work rate and time to exhaustion during muscular exercise, the "critical power" (CP), represents the highest constant work rate that can be sustained without a progressive loss of homeostasis [as assessed using ³¹P magnetic resonance spectroscopy (MRS) measurements of muscle metabolites]. Six healthy male subjects initially completed single-leg knee-extension exercise at three to four different constant work rates to the limit of tolerance (range 3–18 min) for estimation of the CP (mean ± SD, 20 ± 2 W). Subsequently, the subjects exercised at work rates 10% below CP (<CP) for 20 min and 10% above CP (>CP) for as long as possible, while the metabolic responses in the contracting quadriceps muscle, i.e., phosphorylcreatine concentration ([PCr]), P_i concentration ([P_i]), and pH, were estimated using ³¹P-MRS. All subjects completed 20 min of <CP exercise without duress, whereas the limit of tolerance during >CP exercise was 14.7 ± 7.1 min. During <CP exercise, stable values for [PCr], [P_i], and pH were attained within 3 min after the onset of exercise, and there were no further significant changes in these variables (end-exercise values = 68 ± 11% of baseline [PCr], 314 ± 216% of baseline [P_i], and pH 7.01 ± 0.03). During >CP exercise, however, [PCr] continued to fall to the point of exhaustion and [P_i] and pH changed precipitously to values that are typically observed at the termination of high-intensity exhaustive exercise (end-exercise values = 26 ± 16% of baseline [PCr], 564 ± 167% of baseline [P_i], and pH 6.87 ± 0.10, all P < 0.05 vs. <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⁺ concentration and [P_i]), which have been associated with the fatigue process.

fatigue threshold; maximal steady state; exercise tolerance; oxygen kinetics

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