Myocardial Mechanics: Tension-velocity-length Relationships of Heart Muscle.

• The purpose of this paper is to present recent findings that define in greater detail the mechanical behavior of the myocardium. Investigation of the mechanical events associated with myocardial contraction involves at least three parts: 1) study of the contractile elements of the muscle itself, 2) study of the elastic and hydraulic machinery coupling the forces of the contractile elements to the ventricular cavity, and 3) study of the nature of the load imposed on the ventricular cavity by the distal circulation. Although all three parts bear on this presentation, it is with the first of these, namely, the mechanical nature of the contractile element, that this report is primarily concerned. A. V. Hill's model of skeletal muscle* has been adopted tentatively as a prototype for the mechanical behavior of cardiac muscle. A mathematical model consistent with these views is shown schematically in figure 1. It consists of a circumferential arrangement of contractile elements attached to one another by a series of relatively stiff springs. These springs represent the "series elastic component" of the heart. The elasticity represented by the other spring, shown in figure 1 to be running parallel to the contractile elements, represents the "parallel elastic component" of the heart. During systole both elastic systems will be acting in parallel. Since the stiffness of the heart during systole is many times greater than that during diastole, the stiffness of the parallel elastic component is much less than that of the series elastic component. Thus, to simplify analysis