Is stepwise sarcomere shortening an artefact? A response.

Like any other signals, optical signals from muscle fibres inevitably contain noise. The noise here is of predictable character. Because the structure of muscle is periodic, or almost so, translational movement of an illuminated fibre will inevitably give rise to a periodic noise fluctuation as striations pass across the optical field. The frequency of this fluctuation should be linearly related to the speed at which the fibre translates. By mistaking this noise component for the signal, Altringham and colleagues have amply succeeded in confirming this expected relationship, but they have left unanswered the question of the source and nature of stepwise shortening. I show here that: (1) by having obtained records in conditions in which the noise dominated, Altringham et al. have inadvertently confused the steps with the translation-induced fluctuation; and (2) when records are obtained using a method designed specifically to circumvent the effects of translation, the steps still remain in evidence.