Diverse changes in fibre type composition of the human lateral pterygoid and digastric muscles during aging.

The fibre type composition of the superior and inferior portions of the human lateral pterygoid and the anterior and posterior bellies of the digastric muscles of five elderly subjects (mean age 73 years) was studied by morphological and enzyme-histochemical methods. Both muscles showed significant age-related changes in fibre type composition as compared with previous data for young adults. In the lateral pterygoid we observed a large proportion of type IIA fibres, which are rare or absent in young adults, and muscle fibre atrophy and an increased variability in fibre diameter. The digastric muscle of elderly showed a decrease in the proportion of type IIB fibres. The only difference in age-related changes between muscle portions was found in the lateral pterygoid with fibre atrophy in its inferior portion. Both the lateral pterygoid and digastric muscles are known to be active in mandibular depression (jaw opening) and horizontal positioning of the mandible. The present results and previous data from young adults show that the lateral pterygoid and digastric muscles differ not only in fibre type composition, but also in muscular changes following aging. This suggests that, even if they are simultaneously active, they fulfill different, specific tasks in natural jaw function. The differences in age-related changes in fibre type composition between these two muscles indicate that mechanisms underlying their alterations during aging are muscle specific. The results indicate that, although nerve supply and developmental history are essential for fibre composition of skeletal muscles, functional tasks and demands are of major importance.