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.
The work done during fast recoil of active striated muscle (as in a jump) was measured at 2 and 12 °C by making tetanized frog sartorii shorten from about 2 mm above slack length, l 0 , at high speed(6–9 l 0 s −1 )(1) during a state of isometric contraction and (2) after stretching the muscle, while active, at different speeds and by different amounts. The work done increases with the force developed by the muscle according to a sigmoidal curve, having a point of inflexion that is displaced to greater values of force at 12 °C than at 2 °C. Previous stretching leads to an upward shift of this curve, i.e. to an iso-force gain of energy. This gain increases towards a maximum as the speed and extent of stretching are increased, attaining 60–80% of the total work done from a state of isometric contraction; this fraction decreases when stretching begins from lengths smaller than l 0 . The apparent elastic behaviour of muscle is thus described by a set of curves rather than by a single curve. Active muscle behaves as a more rigid structure when it transmits the generated force to an external load (as in an isometric contraction) and as a more compliant structure when, stretched by an external force, it has the opportunity to store external mechanical energy. Note: