[Correlations between feet posture and muscle fiber size in mammals--comparison of the anterior tibial muscle fiber among plantigrade type, digitigrade type and unguligrade type].

The size (micron2) of red muscle fibers (Type 1), white muscle fibers (type 2) and intermediate muscle fibers (type 3) was quantitatively determined in cross-sectional areas of the venter of the anterior tibial muscle in the plantigrade type, digitigrade type and unguligrade type using 122 individuals of 15 species in 5 orders, and correlations between the type of locomotion and muscle fiber size were analyzed from a comparative anatomical viewpoint. Regardless of species, red muscle fibers with slow contraction were small in diameter, intermediate muscle fibers (which are believed to have characteristics of both types 1 and 2) were intermediate in diameter, and white muscle fibers with fast contraction were large in diameter. Muscle fiber size (types 1 through 3) was largest in the orangutan, followed by the brown bear, rat, crab-eating monkey and human, in that order, in the plantigrade type. The size was intermediate in the goat, pig and horse in the unguligrade type, and white-handed gibbon, chimpanzee and rabbit in the plantigrade type. The size was relatively small in the cat, dog, Japanese raccoon dog and Japanese for in the digitigrade type within the same species, muscle fiber size increased in proportion to the increase in body weight. However, there was no correlation between body size and muscle fiber size among different species. The above results suggest that muscle fiber size in the mammalian leg is regulated by anatomical morphology, i.e., plantigrade, digitigrade and unguligrade types, and shows individual adaptive variation.