1. The chemical changes during contractile activity were separated from recovery metabolism in the forearm flexor musculature in normal human subjects using 31P nuclear magnetic resonance (NMR) spectroscopy. Percutaneous, supramaximal twitch stimulation of the median and ulnar nerves was used in combination with temporary ischaemia of the forearm to characterize the summed ATPase activity. The recovery following restoration of blood flow provided a measure of oxidative ATP synthesis activity. These processes were measured based on the dynamics of creatine phosphate (PCr) content. 2. Muscle oxygen stores were depleted using ischaemia without stimulation as indicated by PCr breakdown after 250 +/- 33 s (mean +/- S.D.; n = 5), which provided a measure of the basal metabolic rate (0.008 +/- 0.002 mM s-1, n = 5). 3. The PCr breakdown rate during twitch stimulation of the oxygen-depleted muscle was constant at 1 Hz at 0.15 +/- 0.03 mM PCr per second or per twitch (n = 8). A constant cost per twitch was found from 0.5 to 2 Hz stimulation (depletion of PCr per twitch = 0.15 mM per twitch). 4. No net anaerobic recovery of PCr was found during a 2 min post-stimulation ischaemia. 5. Upon restoration of blood flow, PCr recovery followed an exponential time course with a time constant of 63 +/- 14 s (n = 8). From these recovery rates, the capacity for oxidative phosphorylation was estimated to be 0.4 mM s-1. 6. This experimental approach defines a non-invasive and quantitative measure of human muscle ATPase rate and ATP synthetase rate.