This preclinical study demonstrates that the duration of the active state in myocardial contraction is determined by the amount of shortening of the contractile element, and that force-producing structures do not substantially contribute to series compliance.
Abstract The time course of the active state, the force‐velocity relation and the series compliance were investigated in isolated papillary muscles of the rabbit.—The force‐velocity curve defined at a precise length state of the contractile element and at a given time after the stimulus could be fitted by Hill's equation. The shape of the curve remained constant during the activity period, indicating that the ability to produce motion and the ability to produce tension undergo parallel changes during a myocardial contraction cycle.—The active state, at a given length of the contractile unit, had a slow onset (1/3–1/2 of the entire activity period being needed for attainment of the maximum) and did not form a distinct plateau. The time course of the active state was critically dependent on the degree of extension of the contractile unit. Evidence is presented that the duration of the active state is not definitely programmed at the start of the contraction; its final outcome is determined by the amount of shortening of the contractile element during the activity period.—The stiffness of the series elastic element, at a given tension, was independent of the intensity of the active state. This fact supports the view that the active, force‐producing structures in the cell do not contribute substantially to the series compliance of the cardiac muscle.
Edman et al. (Tue,) studied this question.