31P NMR spectroscopy at 4.7 T has been used in vivo to follow metabolic changes associated with exercise and subsequent recovery in the forearm flexor digitorum superficialis muscle of 14 healthy volunteers. The muscle content in phosphomonoesters at rest provides an index of glycogenolytic activity. Quantitative linear correlations have been shown to link end-of-exercise acidosis to recovery kinetics of phosphocreatine and phosphocreatine/organic phosphate ratio. These linear relationships constitute new metabolic invariants to be used in the study of myopathies and muscle adaptation to exercise.