Enzymic properties of myosin in fast and slow twitch muscles of the cat following cross-innervation.

1. Following cross-innervation of the cat flexor digitorum longus muscle (F.D.L., fast-twitch) and soleus muscle (slow-twitch) the isometric twitch speed of the F.D.L. muscle decreases while the contraction time of soleus increases. The tetanus-twitch ratio of soleus is also increased.2. Myosin or myofibrils from normal or self-reinnervated fast-twitch muscles of the cat split adenosine triphosphate (ATP) more rapidly than myosin or myofibrils from slow twitch muscles.3. Following cross-innervation the myosin and myofibrillar ATPase activity of flexor digitorum longus muscle falls, to become nearly equal to that of normal or self-reinnervated soleus muscle. The ATPase activity of cross-innervated soleus muscle increases only slightly.