Instantaneous Force-velocity-length Determinants in the Contraction of Heart Muscle.

• The inverse relation between force and velocity of shortening comprises one of the most fundamental properties of the contractile system of muscle.This force-velocity relation was first defined for skeletal muscle relative to one initial muscle length. Subsequently, Abbott and Willcie demonstrated that the same inverse relation applies at any instant during the course of contraction if modifications are made to account for instantaneous changes in muscle length as the muscle shortens. Recently, using the cat papillary muscle, the force-velocity relation, relative to initial muscle length, has also been demonstrated in heart muscle."' These studies have allowed a mechanical definition of the manner in which myocardial force generation is altered by changing initial muscle length.' Increasing initial muscle length has been shown to increase the maximum isometric force generated by the muscle (Po), but not to change the maximum velocity of shortening of the unloaded muscle (Vm a i) . 1 0 1 1 In contrast, inotropic interventions which alter the contractile state of the muscle at a constant initial muscle length, such as changing frequency of contraction' or the addition of norepinephrine, have been found to increase Vmai, with or without a change in P0.°n However, the basic question remains whether velocity of shortening of heart muscle at any instant during the course of contraction depends on the muscle length at that instant in time in the same manner that the initial velocity of shortening is determined by the initial muscle length from which the contraction began.