CHANGES IN THE MAXIMUM SPEED OF SHORTENING OF FROG-MUSCLE FIBERS EARLY IN A TETANIC CONTRACTION AND DURING RELAXATION

1. Isotonic shortening velocities at very light loads were examined in single fibres of the anterior tibialis muscle of the frog, Rana tempo raria, using load-clamp recording and slack tests (temperature, 1-3 de grees C; initial sarcomere length, 2.25 mu m). 2. Shortening velocitie s at very light loads (for ce-clamp recording) were found to be higher early in the rise of a tetanic contraction than during the plateau of the contraction. The upper limit of the load at which there was eleva ted shortening velocity early in the contraction was 1.5-5.4% of the m aximum tetanic tension (F-0) depending on the particular fibre. 3. The maximum shortening velocity determined using the slack test method (V -0) was as much as 30% greater early in a contraction than at the teta nic plateau. V-0 was elevated above the plateau level up to about 30 m s after the end of the latent period, which is equivalent to the time required for the force in an isometric contraction to rise to about 30 % of F-0. V-0 is depressed below the plateau value during relaxation a t the cessation of stimulation. 4. Simulation studies show that the cr oss-bridge model of Huxley (1957) predicts the maximum shortening velo city to be greater early in a contraction, when new actin binding site s are becoming activated and new croos-bridge connections are being fo rmed rapidly than during steady-state contraction. The elevated shorte ning velocity in the model is a consequence of new cross-bridges being formed in the pulling configuration, and there being a delay before t he newly added bridges are dragged beyond their equilibrium position s o they begin to retard shortening. The model also predicts that maximu m shortening velocity should be depressed below the plateau level duri ng early relaxation as cross-bridge binding sites are rapidly removed from the active population.