The force-velocity relation and stepwise shortening in cardiac muscle

A series of experiments was carried out to determine the effects of load variation on the character of stepwise shortening. We imposed afterloaded isotonic contractions on rat ventricular trabeculae, and measured the effect of load on pause duration, i.e., on the duration of the periods during which there was no sarcomere shortening. Sarcomere lengths were measured by optical diffraction. Increases of load brought about increases of pause duration; the relation was linear. The relation did not appear to depend on time during contraction, but did depend on sarcomere length: for a given load, pauses were longer at shorter sarcomere lengths. In a supplementary protocol in which we measured the dynamics of the central segment of the muscle during muscle isometric contraction, we found that the velocity of sarcomere shortening during the shortening step was approximately independent of load. These results provide a framework for interpretation of muscle force-velocity relations: diminished velocity at high load may be the result of increased pause durations.