MUSCLE-FIBER COMPOSITION AND INNERVATION RATIOS OF THE EXTENSOR DIGITORUM AND EXTENSOR HALLUCIS BREVIS MUSCLES IN THE CRAB-EATING MACAQUE (MACACA-FASCICULARIS)

The morphological and functional characteristics of the extensor digit orum and hallucis brevis muscles, the sole intrinsic muscles of the do rsum of the foot in the macaque, were investigated through dissection, the examination of muscle fibre composition, and counts of axon numbe rs. The total number of muscle fibers contained within the extensor di gitorum (EHB) and extensor hallucis brevis (EDB) was almost 21,100. Th e population ratios of the three muscle fibre types of white, intermed iate and red were 39, 23 and 38%, respectively, for EDB and 40, 23 and 37%, respectively, for EHB. The mean cross-sectional area of each fib re type (white, intermediate, red) tended to be larger in the EHB (2,0 98, 1,480 and 911 mu m(2)) than in the EDB (1,695, 1,310 and 822 mu m( 2)) and the value were significantly different for white fibres of mal es. The percentage area of the three muscle fibre types and the ratio of the white, intermediate and red areas, were similar between the EHB (55, 23 and 22%, respectively) and the EDB (52, 23 and 25%, respectiv ely). The white area was dominant with the value showing a somewhat la rger figure in the EHB compared to that of the EDB. The number of moto r units was estimated to be 197-234 on average and the innervation rat io was calculated as between 91 and 109. These results suggest that th e muscle fibre composition and innervation ratio of the EHB and EDB, e specially the EHB, might relate to the arboreal locomotion of the crab -eating macaque.