Few subjects in medicine have attracted as much attention from physiologists and clinical investigators alike as has the study of the mechanics of cardiac contraction. Our current understanding of this subject is the result of studies of both isolated muscle and the intact heart. The geometric simplicity of parallel fiber muscle strips and the ability to measure and control specific parameters in isolated muscle preparations have provided the physiologist with a means of systematic analysis of the behavior of the contractile element. Working with isotonic contractions of skeletal muscle strips, Hill described the most fundamental mechanical property of the contractile element: namely, the hyperbolic relation between force generated and the velocity of shortening (1, 2). In his analysis of muscle mechanics, Hill visualized force generation as the stretching of an elastic component that was functionally in series with the contractile element. More recently, Abbott and Mommaerts (3) and Sonnenblick (4) have demonstrated the force-velocity relationship in the cat papillary muscle and have pointed up basic differences between cardiac and skeletal muscle in the behavior of this relationship during specific interventions.Studies of the mechanics of contraction in the intact heart have utilized other measured parameters. The Frank-Starling mechanism (5, 6) has provided us with the most useful index of, the functional state of the intact heart. In the construction of a ventricular function curve, however, the contractile effort of the heart is represented solely by external stroke work, and the total work of the contractile element may not be * Submitted for publication July 9, 1963; accepted February 27, 1964. This work was supported by U. S. Public Health Service research grant H-7139 from the National Heart Institute.t Postdoctoral fellow of the National Heart Institute, U, S. Public Health Service. fully realized (7). Although the time derivatives of stroke work, isometric pressure, and stroke volume further help to characterize the contractile effort of the beating heart, an examination of the fundamental relationship between force and velocity of the contractile element throughout the course of a single systole has not been reported. In a recent discussion of muscle mechanics, however, Fry has presented preliminary data on the instantaneous relation between muscle tension, shortening velocity, and fiber length in the intact heart (8).The purpose of this paper is to describe a method for examining the instantaneous relationship between contractile element velocity and force in the beating heart and to show that the inverse force-velocity relationship observed in isolated muscle is operative in the intact heart. Analysis of this relationship in the intact animal provides a means of interpreting cardiac function in terms of basic muscle mechanics. Methods

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