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 contraction2-5. Stepwise shortening has also been reported recently in relaxed muscle fibres on which length changes were externally imposed4,6,7, 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.
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