An Analysis of Length-Dependent Active Stiffness in Smooth Muscle Strips

Abstract

The measured stiffness of contracting smooth muscle is strongly dependent on the level of developed force. This force-dependent stiffness is a consequence of contractile activity, and it is possible that a portion of it represents the stiffness of the population of attached crossbridges. The relationship between force and stiffness is sensitive to the particular stage of the contraction-and-relaxation cycle (Meiss, 1978), to specific external mechanical constraints imposed on the muscle (Meiss, 1987), and to the length of the muscle when the stiffness is measured (Meiss, 1978; Meiss, 1990). The character of the length-dependent stiffness relationship depends on the mechanical mode of contraction, and interpretation of these effects rests on assumptions regarding how the process of stiffness measurement interacts with changing tissue dimensions. The purpose of this paper is to characterize the difference between the length-dependent stiffness measured in isotonic and isometric contractions. Possible reasons for the differences will be considered, and a tentative model to account for the isotonic length-dependence of stiffness will be proposed.