Objective:To study the composition and distribution of two muscle fiber types in SD rat gastrocnemius muscle in order to learn its function.Methods:Gastrocnemius muscles of both sides of hind limbs were collected from 10 adult SD rats.8-μm-thick cryostat sections were transected and then stained with the method of myosin-ATPase following a procesure modified partially from one that Guth & Samaha reported before.Results:Gastrocnemius muscle from SD rats can be clearly identified with two different fiber types which are typeⅠ(slow twitch fiber)and typeⅡ(fast twitch fiber) by the myosin-ATPase staining method.Fibers of type I are stained white while fibers of type II are stained brown.The distribution pattern of two types of muscle fibers in gastrocnemius muscles is uniform and chessboard-like.The proportion of typeⅡmuscle fibers contained in the muscle is(72.2±6.8)%

much higher than that of typeⅠfibers which is(27.8±2.4)%(P<0.01).Conclusion:Fast-twitch glycolytic(FG) fibers are dominant in SD rat gastrocnemius muscles

which implies that gastrocnemius muscles participate more in fast stepping movement.