Do laser diffraction studies on striated muscle indicate stepwise sarcomere shortening?

ILLUMINATION of striated muscle with a laser beam and measurement of the ensuing diffraction angles is a widely used method for the determination of sarcomere lengths. Pollack et al. 1 recently presented results showing that during internal shortening of single muscle fibres the angle of the diffracted first order beam changes in stepwise fashion. As the most likely explanation for this finding they suggested that the majority of the sarcomeres in the field illuminated by the laser beam, after having started shortening, suddenly cease shortening for several milliseconds before continuing the shortening process. The authors point out that this possibility would impose serious complications on the cross-bridge theory as presently envisaged 2,3 , and in fact, on any proposed molecular theory of contraction. On the basis of diffraction studies using an experimental arrangement similar to that of Pollack et al. we now propose that the steps in the records of sarcomere length during contraction are an expression of slight differences in sarcomere length existing in myofibrillar clusters within the muscle fibre and therefore do not conflict with present contraction theories.