Metabolic fluctuation during a muscle contraction cycle

Gated ³¹P-nuclear magnetic resonance followed the metabolic fluctuation in rat gastrocnemius muscle during a contraction cycle. Within 16 ms after stimulation, the phosphocreatine (PCr) level drops 11.3% from its reference state. The PCr minimum corresponds closely to the time of maximum force contraction. P(i) increases stoichiometrically, while ATP remains constant. During a twitch, PCr hydrolysis produces 3.1 μmol ATP/g tissue, which is substantially higher than the reported 0.3 μmol ATP·twitch^-·g tissue^-1 derived from steady-state experiments. The results reveal that a substantial energy fluctuation accompanies a muscle twitch.