Augmented force output in skeletal muscle fibres of Xenopus following a preceding bout of activity.

1. The effect of a brief period of activity on subsequent isometric tetanic force production was investigated in single muscle fibres of Xenopus laevis. 2. Following a train of ten tetani separated by 4 s intervals, tetanic force was significantly augmented by about 10%. The tetanic force augmentation persisted for at least 15 min and then slowly subsided. A similar potentiation was seen following trains of five and twenty tetani. 3. During the period of tetanic force potentiation, tetanic calcium was reduced by more than 30%, and intracellular pH was reduced from 7.15 +/- 0.07 to 7.03 +/- 0.11 (n = 4). 4. Fibre swelling was greatest at 1 min and then subsided over 15-20 min and possibly accounted for a small part of the observed force potentiation. 5. A reduction in the inorganic phosphate (P1) concentration of more than 40% was found in fibres frozen in liquid nitrogen at the peak of force potentiation compared with resting fibres. 6. It is concluded that the augmentation of tetanic force found after a brief preceding bout of activity is due to a reduction in inorganic phosphate. This mechanism may underlie the improved performance observed in athletes after warm-up.