Individual Variation in Myofiber Type Composition in the Triceps Surae and Flexor Digitorum Superficialis Muscles of Japanese Macaques

Abstract

Individual variation in composition of myofiber types (SO, FOG, and FG) was examined in the m. triceps surae and m. flexor digitorum superficialis of male and female Japanese macaques. The m. soleus generally had more SO myofibers than FOG and FG myofibers. The m. gastrocnemius and m. flexor digitorum superficialis had more FG myofibers than the other myofiber types. No sex-related differences in composition of myofiber types were noted in the muscles. Great differences in myofiber type percentages existed among the animals. The proportion of SO myofibers ranged from 32.8 to 95.9% in the m. soleus, from 8.1 to 50.5% in the m. gastrocnemius caput mediale, from 6.5 to 43.8% in the caput laterale, and from 3.2 to 44.7% in the m. flexor digitorum superficialis. Interindividual differences in myofiber type composition may be determined partially by genetic factors and be due partially to differences in capacity for changing myofiber types to meet postural or locomotory requirements.