Single-skinned human muscle fibers: activation by calcium and strontium.

Single-skinned fibers were prepared from human skeletal muscle. Tension generation was measured in solutions containing different amounts of free calcium or strontium ion. Fibers were classified into two types, on the basis of distinct differences in their regulation by strontium. One group developed maximal tension in 3 x 10-5 M free strontium, whereas the other developed minimal or no tension in the same concentration of strontium. It seemed most probable that the former represented type 1 (slow twitch) fiber and the latter type 2 (fast twitch) fiber because of similar patterns observed in guinea pig soleus and extensor digitorum longus muscle fibers.