The effects of MM-creatine kinase deficiency on sustained force production of mouse fast skeletal muscle.

Twitch force production was normal in muscles of mice which lack MM-creatine kinase, but the tetanic force:twitch force ratio was lower than in control muscles (3.60 vs 4.27; P < 0.05). In a series of repeated tetanic contractions the force in the second contraction was already markedly depressed (20-50%), while subsequently only small changes were observed. The effect was greater in exercise that would require a higher metabolic peak flux. The depressed force production was not accompanied by a slowing of relaxation, indicating that enough ATP was present to sustain myofibrillar ATPase and Ca(2+)-ATPase activity.