Segmental length changes in stimulated frog sartorius muscle during dynamic mechanical responses

The nonuniformity of dynamic mechanical responses along the length of stimulated frog sartorius muscle was examined by recording the length changes of muscle segments by means of streak photography. During isometric force development, the central or the pelvic end segment tended to shorten due to the stretching of the other segments. In muscles showing a marked nonuniformity in the segmental length changes occurring during isometric force development, there was also a marked nonuniformity in the segmental length changes occurring during stretches or releases applied at the plateau phase of an isometric tetanus. Marked nonuniform segmental length changes were also observed during afterloaded twitches and tetani. These results indicate that great care should be taken of the muscle segmental nonuniformity in heat and X-ray diffraction studies in which measurements are made using data obtained from a limited region of muscle. During the course of slow stretches or releases applied to tetanized muscles, the tibial and segment exhibited much smaller relative lengthening or shortening than the other segments when it had been appreciably stretched by the other segments during isometric force development. This might be associated with the formation of "locked-on" cross-links in the stretched segment.