Similar metabolic perturbations during all-out and constant force exhaustive exercise in humans: a 31P magnetic resonance spectroscopy study

It is not possible to attain a metabolic steady state during exercise above the so-called critical force or critical power. We tested the hypothesis that the muscle metabolic perturbations at the end of a bout of maximal isometric contractions, which yield a stable end-test force (equal to the critical force), would be similar to that at task failure following submaximal contractions performed above the critical force. Eight healthy subjects (four female) performed isometric single knee-extension exercise in the bore of a 1.5 T superconducting magnet on two occasions. Following familiarization, subjects performed the following exercises: (1) 60 maximal contractions (3 s contraction, 2 s rest); and (2) submaximal contractions (the same contraction regime performed at 54 ± 8% maximal voluntary contraction) to task failure. Phosphocreatine (PCr), inorganic phosphate (Pi) and diprotonated phosphate (H2PO4?) concentrations and pH were determined using 31P magnetic resonance spectroscopy throughout both tests. During the maximal contractions, force production fell from 213 ± 33 N to reach a plateau in the last 30 s of the test at 100 ± 20 N. The muscle metabolic responses at the end of each test were substantial, but not different between conditions: [PCr] was reduced (to 21 ± 12 and 17 ± 7% of baseline for maximal and submaximal contractions, respectively; P = 0.17), [Pi] was elevated (to 364 ± 98 and 363 ± 135% of baseline, respectively; P = 0.98) and pH reduced (to 6.64 ± 0.16 and 6.69 ± 0.17, respectively; P = 0.43). The [H2PO4?] was also elevated at the end of both tests (to 607 ± 252 and 556 ± 269% of baseline, respectively; P = 0.22). These data suggest that the exercise-induced metabolic perturbations contributing to force depression in all-out exercise are the same as those contributing to task failure during submaximal contractions.