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. ADDITIONAL KEY WORDS contractility contractile state myocardium force-velocity relation • In recent years a useful approach to evaluation of the contractility of the myocardium has been based on the fact that the velocity of muscle shortening decreases with increasing afterload—the force-velocity relation (1-3). However, such measurements are generally taken from the peak velocities of shortening of nontetanized twitches of heart muscle. Thus this analysis considers neither the instantaneous length of contractile elements during contraction nor the effects of the various times after the stimulus when measurements are made. To overcome these limitations, the instantaneous relations between force, velocity , and length or between force, velocity, and time have been considered (4), and a given contractile state of the myocardium has been characterized by the surface, or envelope, created by the instantaneous relations between force, velocity of shortening, and muscle length. To correct for problems of time, quick-release experiments using the methods of Jewell and Wilkie (5) for skeletal muscle have also been performed. Immediately following the release of the isometrically con