Stepwise shortening in unstimulated frog skeletal muscle fibres.

We investigated the dynamics of sarcomere length change during imposed stretches and releases of unstimulated single fibres of frog skeletal muscle. Three independent methods were used: an on-line method in which sarcomere length is computed from the striation pattern; laser diffraction; and a segment length tracking device. During steady ramp releases and stretches, both sarcomere and segment length changes occurred in stepwise fashion; i.e. periods of pause were interspersed between periods of rapid shortening. The above result indicates that activation of the fibre is not required to elicit stepwise length changes. Increasing the ramp velocity caused the steps to increase in size and the pauses to decrease in duration. Ramp releases and stretches were imposed at each of several initial sarcomere lengths up to 4.0 microns. Stepwise length changes were observed at all lengths, and their size was independent of initial sarcomere length. The observation of stepwise length changes beyond overlap indicates that the underlying mechanism probably does not lie in synchronous action of cross-bridges; an alternative hypothesis is advanced.