THE MECHANICAL-PROPERTIES OF FAST AND SLOW SKELETAL-MUSCLES OF THE MOUSE IN RELATION TO THEIR LOCOMOTORY FUNCTION

The mechanical properties of soleus and extensor digitorum longus (EDL ) muscles from the mouse were studied using the work loop technique. U nder optimum conditions, the EDL produced a maximum mean power output of 107Wkg(-1) at a cycle frequency of 10 Hz. In comparison, the maximu m mean power output of the soleus was 34Wkg(-1) at 5 Hz cycle frequenc y. Video analysis of mice determined the stride frequency range to be from 2.87Hz at a walk to 8.23 Hz at a flat-out gallop, with the trot-t o-gallop transition occurring at 5.89 Hz. In vivo EDL electromyogram ( EMG) activity is recorded primarily during shortening and the muscle o perates in a power-generating mode. The soleus is close to isometric w hen EMG activity is recorded, but mechanical activity persists into th e shortening phase. Both muscles are likely to operate over cycle freq uency ranges just below, or at, those yielding maximal power. Soleus a nd EDL produced maximal power output irt vitro when operating at mean sarcomere lengths of 2.58 mu m and 2.71 mu m respectively. These lengt hs are slightly above the plateau of the length-force curve predicted for rat leg muscle (2.3-2.5 mu m). The sarcomere length ranges used in vivo by the soleus and EDL were determined, by fixing muscles in the extreme active positions predicted from video and cine analysis, to be 2.28-2.57 mu m and 2.49-2.88 mu m respectively. These ranges are both close to those shown to yield maximum power output in vitro and to th e plateau of the sarcomere length-force curve.