Special histochemical muscle-fibre characteristics of the human lateral pterygoid muscle

Abstract

The histochemical fibre-type profile was analyzed in 10 human lateral pterygoid muscles from young adults with complete dentitions and normal intermaxillary relations. The distributions of various types of fibre in the lateral pterygoid differed from those of normal human limb and trunk muscles as well as from those of human masseter and temporal muscles. There was a marked predominance of type I fibres, a high frequency of adenosine triphosphatase (ATPase) intermediate fibres and type IIC fibres and lack of type IIA ones. The muscle fibres, especially the type II fibres, were smaller than those in limb and trunk muscles. There was marked intra-muscular variability in both the proportion and diameter of the types. Type I fibres were larger in diameter than the others and occupied, on average, 81 per cent of the overall fibre crass-sectional area. Type II and the ATPase intermediate fibres were larger in males than in females. There was no difference in type I fibre diameter or fibre-type proportion between females and males. No difference in fibre diameter was observed between the two heads of the muscle, but the proportions of fibre types differed significantly in four muscles. The fibre-type composition in the adult lateral pterygoid muscle indicates a capacity for endurance during continuous work at relatively low forces and accords with the stabilizing effect on the mandibular joint attributed to this muscle.