Sarcomere dynamics during isotonic velocity transients in single frog muscle fibers.

If the load on a tetanized fiber is abruptly changed to a new steady value, the ensuing fiber length change shows the well-known "isotonic velocity transient," in which the velocity oscillates before settling at some steady value. We studied sarcomere dynamics during these transients using two methods: optical diffraction and a segment-length method. Our principal aim was to determine whether these transients might be a reflection of the fact that sarcomere shortening is often found to be stepwise. We found that pauses in sarcomere shortening occurred during the low-velocity phases of the transient and that steps of sarcomere shortening occurred during the high-velocity phases. Thus the isotonic transient appears to arise from the steps. In addition to the isotonic transient, we studied the well-known isometric transient, in which fiber length is abruptly changed, and ensuing tension response is measured. Again, we found that the transient may be a reflection of the stepwise shortening pattern.