In frog muscle fibres, tetanically stimulated at a sarcomere length of about 2 micron, stretched at a velocity of 1 lengths-1 and released against a force equal to the maximum isometric, P0, a phase of rapid isotonic shortening takes place after release. As the amplitude of the stretch is increased from 1.5 to 9% of the initial length: (1) the amount of rapid isotonic shortening increases up to 9–10 nm per half sarcomere and (2) the stiffness of the fibre (an indication of the number of bridges attached) decreases to a value about equal to that measured during an isometric contraction. If a 5–10 ms delay is left between the end of stretch and release, the amount of rapid isotonic shortening increases to about 12 nm hs-1. A 300–500 ms delay, however, results in a decrease in rapid isotonic shortening to about 5 nm hs-1 and also results in a velocity transients against P0 that are similar to those described during release from a state of isometric contraction. It is concluded that the force attained after large, fast stretches is due to a greater force developed by each bridge and not to a greater number of bridges. After the elastic recoil (when the force is suddenly reduced to P0), these strained bridges are able to shorten by about 12 nm hs-1, suggesting that, during and immediately after stretching, they are charged to levels of potential energy greater than those attained in an isometric contraction.

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