Gated P-31-nuclear magnetic resonance followed the metabolic fluctuati
on in rat gastrocnemius muscle during a contraction cycle. Within 16 m
s after stimulation, the phosphocreatine (PCr) level drops 11.3% from
its reference state. The PCr minimum corresponds closely to the time o
f maximum force contraction. P-i increases stoichiometrically, while A
TP remains constant. During a twitch, PCr hydrolysis produces 3.1 mu m
ol ATP/g tissue, which is substantially higher than the reported 0.3 m
u mol ATP.twitch(-1).g tissue(-1) derived from steady-state experiment
s. The results reveal that a substantial energy fluctuation accompanie
s a muscle twitch.