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