Effects of stretch on work from fast and slow muscles of mice: damped and undamped energy release.

Stretching active muscle increases the work performed during subsequent shortening. The effects of a preceding stretch on work done by the undamped or lightly damped series compliance (SC) and by the contractile component (CC), which includes cross bridges and damped elements, were assessed using mouse soleus (slow) and extensor digitorum longus (fast) muscles with limited tendon. Increasing stretch amplitude (0-10% fibre length) increased work done by the SC up to a limit, but did not effect work done by the CC. Increasing stretch velocity (10-100% Vmax) had almost no effect on work done by either component. Increasing the delay between the end of stretch and onset of shortening (0-60 ms) caused a decrease in SC work, with no effect on CC work. Recoil of the SC was responsible for 50-70% of the total work done during shortening after stretch. Usually only 10-40% of the energy imparted during the stretch was recovered as work during subsequent shortening; large stretches and long delays between stretch and shortening further reduced this recovery by one third to one fifth. Results are interpreted in the context of a loss of energy stored in the SC owing to forcible detachment of cross bridges with large stretches and cyclic detachment with long delays.