Force-velocity characteristics for calcium-activated mammalian slow-twitch and fast-twitch skeletal fibers from the guinea pig.

Twitch fiber bundles from a slow (soleus) and a fast (extensor digitorum longus) mammalian muscle after mild chemical treatment were activated with calcium and relaxed in calcium-free solution. Like the electrically activated whole muscles, the force-velocity relationship was such that, at each relative load, the steady speed of shortening for the fast fibers was about two times greater than that for the slow twitch fibers. The duration of pre-steady motion in the two types of fibers was the same. The data provide direct evidence that the difference in the shortening characteristics of the two types of fibers is due to differences in their cross-bridge properties.