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

Abstract

Twtich fiber bundles from a slow (soleus) and a fast (extensor digitorum longus) mammalian muscle after mild chemical treatment were activated with Ca and relaxed in Ca-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 2 times greater than that for the slow twitch fibers. The duration of pre-steady motion in the 2 types of fibers was the same. The data provided direct evidence that the difference in the shortening characteristics of the 2 types of fibers was due to differences in their cross-bridge properties.