Calcium sensitivity of fast- and slow-twitch human muscle fibers.

Muscle fibers from the lateral gastrocnemius muscles of five normal adult males were chemically skinned (sarcolemma disrupted) and each fiber was divided into two parts for histochemical determination of fiber type and calcium-induced tension measurements. The calcium concentration associated with half-maximal tension was 2.5 microM for fast-twitch and 1.3-1.4 microM for slow-twitch fibers. Fast-oxidative-glycolytic (type 2A) and fast-glycolytic (type 2B) fibers had similar calcium sensitivities. Maximum tensions (kg/cm2) were 2.24 for fast-twitch and 2.19 for slow-twitch fibers.