The effect of initial length on the shortening velocity of cat hind limb muscles.

The effect of initial muscle length on the speed of shortening at different relative loads has been determined for the soleus and flexor digitorum longus (FDL) muscles of the cat. Isometric tetanic force-length relationships for both muscles were similar to those shown previously. The functional length range for FDL occurred at relatively long lengths, from optimum (100%) to 135% of optimum length; however, soleus worked at relatively short lengths from 60% to 110% of optimum length. In FDL the speed of shortening at any given load was relatively constant within the functional range, but at very short muscle lengths the speed of shortening declined. Soleus also showed a decline in the speed of shortening at all loads at short muscle lengths, which included the functional working range of the muscle. Speed of shortening at any given load was maximal at optimum length but tended to decline at low loads and long muscle lengths. It is concluded that in FDL even when the toes are at the extremity of their range, speed of muscle shortening is unaffected. Soleus may be relatively disadvantaged because its functional range extends over short muscle lengths. The results indicate that soleus is capable of making a significant contribution in standing and a slow walk, but that at faster gaits the contribution of soleus may be negligible.