This study compared biochemical and 31P-nuclear magnetic resonance (NMR) determinations of energy metabolites during isometric contractions of the human calf muscle at various exercise intensities. Seven male subjects performed one-legged isometric contractions at a work load of 28, 64, and 90% of maximal voluntary contraction force (28-, 64-, and 90%-CON, respectively) for 3 min, 40 s, and 40 s, respectively, in a magnet and in an exact model of the magnet with an arrangement for rapid muscle biopsy sampling from the gastrocnemius. The decrease in phosphocreatine (CrP) determined by NMR was 20, 33, and 71% for 28%-, 64%-, and 90%-CON, respectively. These decreases were the same as those determined biochemically (25, 34, and 61%, respectively). Muscle CrP 1 min after 90%-CON was also found to be similar between NMR and biochemical determinations (88 and 74% of resting value, respectively). Although no significant change in muscle ATP was found by NMR, a decrease of 29% was observed biochemically at 90%-CON. The ratio between muscle CrP and ATP was the same between NMR and biochemical determinations except for 90%-CON (1.98 and 0.78, respectively). The increase in muscle ADP determined by NMR was two-, five-, and eightfold higher than that found biochemically for 28%-, 64%-, and 90%-CON, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)