A plausible mechanism of muscle stabilization in stall conditions

Abstract

We address the well-known limitation of the Huxley and Simmons 1971 (HS) model. It is a statement that at physiological value of stiffness in the actomyosin complex, the distribution of the myosin motors becomes microscopically uniform (all the motors are either in pre- or post-power stroke conformation) after an infinitesimal displacement from the stall (isometric contractions) conditions. Such uniform behavior at the fiber level would generate a negative slope in the $T_2-\delta$ relationship (in the nomenclature of the HS paper), not observed experimentally. This negative slope means inhomogeneity of the macroscopic sarcomere configuration, which is also not observed. To address this controversial prediction of the HS theory, we explore the possibility that the slope of the $T_2-\delta$ curve is, in fact, positive due to an interaction between neighboring cross-bridges. We show that such interaction can potentially destabilize the uniform configurations (all pre or all post) by making the non-uniform configurations energetically preferable. We argue that, despite the presence of other factors, which can in principle also ensure the microscopic inhomogeneity of cross-bridge configurations, the implied interaction is an important player in muscle mechanics.