Introduction

Abstract

In the sliding filament model of muscle contraction (A.F. Huxley Prog. Biophys. biophys. Chem. 7: 255–318, 1957), force generation and shortening result from the alternate formation and breaking of cross-links between the thick and thin filaments. It is well known that relaxed muscle is highly extensible, while contracting muscle becomes considerably stiffer. This change is explained as being due to the formation of cross-links between the filaments. Muscle stiffness is normally expressed as ΔP/ΔL, where ΔL is a rapid change in muscle length, while ΔP is the coincident change in tension, and is generally believed to be proportional to the number of cross-links in each sarcomere. Bressler’s contribution examines stiffness using both stretches and releases, while Cecchi’s paper is concerned with measurement of muscle stiffness with sinusoidal length changes in the kHz region.