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The Equivalent Strap Muscle of the Left Ventricle: A Model for Describing Myocardial Force and Length, and Mean Endocardial Fiber‐Shortening Rate in Man

SUMMARY The force exerted by cardiac muscle usually has been estimated as wall stress, i.e., force per unit area of cross-section. This calculation fails to allow for the decrease in number of muscle fibers per square centimeter as the fibers shorten and become thicker during systole. To describe better the behavior of actual muscle fibers, a model composed of fibers has been identified and examined. It permits the fibers forming a left ventricle to be uncurled into a straight or strap-shaped muscle equivalent to the actual ventricular muscle mass. This transform allows quantitative conversion of volumetric and pressure data into linear data of force, velocity, and length (more exactly: equivalent myocardial force, mean endocardial velocity of fiber-shortening, and equivalent endocardial fiber-length). Data from 17 subjects (fifteen with coronary disease) show the left ventricles equivalent to strap muscles exerting a maximum force of 2.1 to 4.8 kg and having end-diastolic lengths of 18 to 24.5 cm. Corresponding end-systolic lengths range from 11 to 22 cm and fiber shortening rates from 0.52 to 1.69 lengths/sec. Algebraic analysis, for spherical and elongated (cylindrical) shapes, suggests that different fiber arrangements in the walls may be equivalent to each other. The inward force exerted by each circular loop of fiber is the same, regardless of diameter, so it is possible that the outer and inner fibers of a thick-walled model may make equal contributions to myocardial force (although, of course, the outer fibers will be longer).