Force-Velocity-Length-Time Relations of the Contractile Elements in Heart Muscle of the Cat

The instantaneous force-velocity-length relations of the contractile elements of the cat papillary muscle have been obtained by determining the phase-plane trajectories of velocity of shortening relative to length during isotonic contractions and correcting these measurements for the series elastic extension during the isometric phase of contraction. The load-extension curve of the series elastic component was obtained by quick release. The velocity-length relations for a given load were independent of initial muscle length and largely independent of the time after stimulation. Velocity-length traces at varying lengths diverged only late in contraction because of a fall in the intensity of the active state. Thus the surface created by the instantaneous force-velocity-length relations serves to define a given contractile state for the contractile element of heart muscle. Further, the linear length-tension relations observed for the contractile element permits an estimation of the maximum development of isometric force with the creation of truly hyperbolic force-velocity curves. Limitations of velocities obtained following quick releases have also been noted.