In vivo 31P NMR spectroscopic study of forearm wrist flexor muscles was performed in two groups of volunteers composed respectively of 6 sedentaries and 6 canoe athletes. A continuous isometric contraction of endurance exercises was adopted in oder to assess the endurance capacity and recovery potential of skeletal muscles. Differences in high energy phosphorus metabolism beetween the sedentaries and athlets were evaluated with and emphasis on the intracellular pH and Pi/PCr ratio as indicators of high energy phosphorus metabolism. There were no differences of baseline pH and Pi/PCr ratio between the two groups. The athletes sustained the exercise at a more acidic intracellular pH and at a higher Pi/PCr ratio of intracellular conditions for an all-out than did the sedentaries. The recovery rate of pH showed no difference between the two groups. There was a tendency of faster recovery of Pi/PCr in athletes showing half recovery time (T1/2) of 39.0$pm$3.0 seconds as compared to t at of sedentaries (55.7 $pm$7.5 seconds). The recovery rate of Pi/PCr as a function of Pi/PCr ratio at a given period of time was significantly faster in athletes than in sedentaries (P<0.001). The correlation coefficient of the recovery rate of Pi/PCr against the Pi/PCr ratio was 0.985 and 0.914 respectively for the athletes and sedentaries. The pH and the Pi/PCr ratio at an all-out state can be used as indicators of encurance capacity and the recovery rate of Pi/PCr, as a reovery potenital of skeletal muscles.