Mitochondrial regulation of phosphocreatine/inorganic phosphate ratios in exercising human muscle: a gated 31P NMR study.

31P NMR is used to determine the relationship between work output in the exercising human forearm and the steady-state capability of oxidative phosphorylation as measured by the phosphocreatine/inorganic phosphate ratio (PCr/Pi). Exercise intensities (one contraction per 5 sec) permitting comfortable continuation of activity for greater than 1 hr produced PCr/Pi of about 1 for a subject of moderate training. Linear relationships between work rate per unit volume of muscle and the 5-min mean PCr/Pi were found for the subject's left and right arms. The protocol affords sensitive criteria of muscle performance in normal subjects and of biochemical or vascular disease in abnormal subjects. The Pi, PCr, and ATP levels found by 31P NMR represent the initial values in the cycle of contraction and relaxation which permit restitution of resting state 4 prior to the next contraction and the continuation of steady-state work performance.