AbstractThe phosphagen cost of maximal isometric twitch and tetanic contractions in rat gastrocnemius muscle was measuredin situby31P NMR with acquisitions gated to precise time points after a brief (<10 s) burst of contractions. Alteration of twitch stimulation frequency did not affect the energy cost per contraction. Pooled results of four twitch rates from 1 to 8 Hz produced an average cost of 0.257±0.012 μmol ATP/g/twitch. This value was compared with the initial twitch cost estimated from the time zero derivative of an exponential fit of averaged scan phosphocreatine (PCr) data from a previous study of 8 min of contractionin situat 0.75 Hz. Agreement of the two estimates validates the use of the fitting/derivative method to assess energy cost and confirms the monoexponential character of the PCr time course. Evaluation of muscle pH changes demonstrated that all ATP use during the brief twitch contraction bouts could be accounted for by PCr hydrolysis. Results also showed a brief delay in onset of PCr recovery, consistent with the response time of oxidative systems. Tetanic contractions of 100 ms duration at 100 Hz required 2.17±0.07 μmol ATP/g muscle/contraction, in general agreement with estimates reported using a variety of other meth