Storage and release of mechanical energy by contracting frog muscle fibres

1. Stretching a contracting muscle leads to greater mechanical work being done during subsequent shortening by its contractile component; the mechanism of this enhancement is not known. 2. This mechanism has been investigated here by subjecting tetanized frog muscle fibres to ramp stretches followed by an isotonic release against a load equal to the maximum isometric tension, T(o). Shortening against T(o) was taken as direct evidence of an absolute increase in the ability to do work as a consequence of the previous stretch. 3. Ramp stretches (0.5-8.6% sarcomere strain, confined to the plateau of the isometric tension-length relationship) were given at different velocities of lengthening (0.03-1.8 sarcomere lengths s-1). Isotonic release to T(o) took place immediately after the end of the ramp, or 5-800 ms after the end of the largest ramp stretches. The length changes taking place after release were measured both at the fibre end and on a tendon-free segment of the fibre. The experiments were carried out at 4 and 14 degrees C. 4. After the elastic recoil of the undamped elastic elements, taking place during the fall in tension at the instant of the isotonic release, a well-defined shortening took place against T(o) (transient shortening against T(o)). 5. The amplitude and time course of transient shortening against T(o) were similar at the fibre end and in the segment, indicating that it is due to a properly of the sarcomeres and not due to stress relaxation of the tendons. 6. Transient shortening against T(o) increased with sarcomere stretch amplitude up to about 8 nm per half-sarcomere independent of stretch velocity. 7. When a short delay (5-20 ms) was introduced between the end of the stretch and the isotonic release, the transient shortening against T(o) did not change; after longer time delays, the transient shortening against T(o) decreased in amplitude. 8. The velocity of transient shortening against T(o) increased with temperature with a temperature coefficient, Q10, of approximately 2.5. 9. It is suggested that transient shortening against T(o) results from the release of mechanical energy stored within the damped element of the cross-bridges. The cross-bridges are brought into a state of greater potential energy not only during the ramp stretch, but also immediately afterwards, during the first phase of stress relaxation.