Effects of abrupt load alterations on force-velocity-length and time relations during isotonic contractions of heart muscle: load clamping.

1. Abrupt alterations in load (load-clamping) have been imposed on cat papillary muscles during the course of isotonic shortening, between the onset of shortening and peak shortening.2. For any given total load, whether imposed during the course of shortening or before stimulation, the velocity of shortening is determined solely by the instantaneous length, and not by the sequence of length and tension changes through which it arrived at that length.3. This unique force-velocity-length relation is independent of time from just after the onset of shortening until just prior to peak shortening.4. These results suggest that a steady state exists for the maximum intensity of active state in heart muscle over a major portion of the time during which isometric force is rising, and that heart muscle always senses total load while shortening.