Energetics of relaxation in frog muscle

1. Changes in the levels of phosphocreatine (PCr) and ATP during relaxation after tetanic contraction have been observed and compared with the heat + work production during the same period. 2. The results were the same using 2 sec or 15 sec tetani and using 15 or 30 Hz stimulation. 3. During relaxation the mean chemical change (of ATP + PCr) was −0·105 ± 0·078 μmol/g muscle ( n = 53). This change is less than would be required to explain the heat + work produced. 4. The PCr split during both 2 sec and 15 sec tetani (up to the last stimulus) was less than would be required to explain the heat + work produced. This confirms previous experiments. 5. It is concluded that the discrepancy between chemical change and heat + work production does not diminish during relaxation but actually increases.