Is stepwise sarcomere shortening an artefact?

Abstract

A report¹ in 1977 raised the intriguing possibility that sarcomere shortening in muscle may occur in a stepwise fashion, in which episodes of shortening are interrupted by periods of little or no movement. This was taken by its authors to imply the synchronous activity of cross-bridges over a large volume of tissue—behaviour which cannot easily be reconciled with commonly accepted views of muscle contraction^2–5. Stepwise shortening has also been reported recently in relaxed muscle fibres on which length changes were externally imposed^4,6,7, and that system has allowed us to define more rigorously the circumstances in which stepwise shortening is observed. Here we report a high correlation between the frequency of ‘steps’ or ‘pauses’ and the translation velocity of the fibre past the measuring system, suggesting that stepwise shortening is not a physiological property of muscle but an instrumentation artefact.