Mechanical transients initiated by ramp stretch and release to P0 in frog muscle fibers

Single fibers from the tibialis muscle of Rana temporaria were subjected to ramp stretches during tetanic stimulation at a sarcomere length of ~2 μm. Immediately after the stretch, or after different time delays, the active fiber was released against a constant force equal to the isometric force (P0) exerted immediately before the stretch. Four phases were detected after release: 1) an elastic recoil of the fiber's undamped elements, 2) a transient rapid shortening, 3) a marked reduction in the velocity of shortening (often to 0), and 4) an apparently steady shortening (sometimes absent). Increasing the amplitude of the stretch from ~2 to 10% of the fiber rest length led to an increase in phase 2 shortening from ~5 to 10 nm per half-sarcomere. Phase 2 shortening increased further (up to 14 nm per half-sarcomere) if a time interval of 5-10 ms was left between the end of large ramp stretches and release to P0. After 50- to 100-ms time intervals, shortening occurred in two steps of ~5 nm per half-sarcomere each. These findings suggest that phase 2 is due to charging, during and after the stretch, of a damped element, which can then shorten against P0 in at least two steps of ~5 nm/half sarcomere each.