Muscle metabolic responses to exercise above and below the “critical power” assessed using31P-MRS

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 (31)P 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 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 (31)P-MRS. All subjects completed 20 min of CP exercise was 14.7 +/- 7.1 min. 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.