Myocardial Force-Velocity Relations Studied in Intact Unanesthetized Man*

In 1938, A. V. Hill demonstrated that the inverse relationship between the force generated and the velocity of shortening constitutes one of the most fundamental mechanical properties of skeletal muscle (1). More recently it has been shown that a similar reciprocal relationship be- tween force and velocity also exists in the isolated cat papillary muscle (2-5). Moreover, at any given muscle length, various positive inotropic interventions, such as augmentation of heart rate (2, 3), administration of norepinephrine (3, 4), Ca++ (3), or digitalis (6), shift the force-velocity relation so that the velocity of shortening is greater at any given load and the maximal velocity of the unloaded muscle is increased. This type of shift in the force-velocity relation is interpreted to reflect an augmentation of the contractile state of the myocardium. The ability to alter the re- lationship between the force generated and the velocity of shortening in heart muscle is in con- trast with the relative stability of the forcevelocity relation in skeletal muscle.