Distribution of myofiber types in the hip and thigh musculature of sheep.

Abstract

The composition of muscles by myofiber type is associated with their locomotory or postural functions. In the present study the composition of the hip and thigh musculature of sheep by myofiber types and the differences in their distribution were examined. Myofibers were classified into type I, IIA, and IIB myofibers by differences in myosin ATPase and NADH tetrazolium reductase (NADH-TR) activity. The vastus intermedius muscle consisted only of type I myofibers, which exhibit weak alkali-stable myosin ATPase and strong NADH-TR activity. The gluteus accessorius and profundus muscles had more than 50% type I myofibers. The other muscles had less than 50% type I myofibers as a whole. Type I myofibers were concentrated in the deep portions of the gluteus and quadriceps femoris muscles, which extend the hip and stifle joints, and of the pectineus muscle. They were scattered evenly in the caudally situated locomotory muscles in the thigh. Type IIA myofibers, characterized by strong alkali-stable myosin ATPase and NADH-TR activity, showed little difference in distribution in the hip and thigh muscles. Type IIB myofibers, characterized by strong alkali-stable myosin ATPase and weak NADH-TR activity, were distributed more in the cranial, caudolateral, and caudomedial portions than in the middle portions of the thigh. The distribution of type IIB myofibers is suited to powerful flexion and extension of the thigh and leg. In the hip and thigh musculature, it appears that type I myofibers are effectively distributed to maintain a standing posture without diminishing the propulsive force of the hindlimb.