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1. Phosphocreatine (PCr) and intracellular pH detected by 31 P NMR in the gastrocnemius and soleus muscles were evaluated in order to compare the anaerobic ATP costs of voluntary and electrically induced exercise. Continuous isometric contraction at 40% of maximum force and repeated isometric contractions at approximately 75% of maximum force (contraction plus relaxation period of 0.5 s plus 2 s) were studied.