STORAGE AND RELEASE OF MECHANICAL ENERGY BY CONTRACTING FROG-MUSCLE FIBERS

1. Stretching a contracting muscle leads to greater mechanical work be ing done during subsequent shortening by its contractile component; th e mechanism of this enhancement is not known. 2. This mechanism has be en investigated here by subjecting tetanized frog muscle fibres to ram p stretches follow ed by an isotonic release against a load equal to t he 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% sar comere 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 imme diately after the end of the ramp, or 5-800 ms after the end of the la rgest ramp stretches. The length changes taking place after release we re 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. Afte r the elastic recoil of the undamped elastic elements, taking place du ring the fall in tension at the instant of the isotonic release, a wel l-defined shortening took place against T-o (transient shortening agai nst T-o). 5. The amplitude and time course of transient shortening aga inst T-o were similar at the fibre end and in the segment, indicating that it is due to a property of the sarcomeres and not due to stress r elaxation of the tendons. 6. Transient shortening against T-o increase d 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, th e transient shortening against T-o did not change; after longer time d elays, the transient shortening against T-o decreased in amplitude. 8. The velocity of transient shortening against T-o increased with tempe rature with a temperature coefficient, Q(10), of similar to 2.5. 9. It is suggested that transient shortening against T-o results from the r elease of mechanical energy stored within the damped element of the cr oss-bridges. The crossbridges are brought into a state of greater pote ntial energy not only during the ramp stretch, but also immediately af terwards, during the fast phase of stress relaxation.