Estimating the Functional Axis of the Primate Foot Using the Distribution of Plantar Muscles

Eishi Hirasaki, Hiroo Kumakura

Morton (1922) used the longest metatarsal, which he assumed functions as a lever during locomotion, to define the functional axis of the primate foot. In humans and apes, the functional foot axis lies on the second digit, whereas that of nonhominoid anthropoids is mostly on the third digit, suggesting that a medial shift of the functional axis occurred during primate foot evolution. Myological observations support this idea; the dorsal interossei of the human foot are arranged around the second digit, whereas those of nonhominoid anthropoids are around the third digit. However, it is still unclear when, why, and how such a change in foot musculature occurred. In addition, there is inconsistency among the limited number of studies that have examined foot musculature in apes. We examined modifications in the interosseous muscles of the chimpanzee, gibbon, spider monkey, and Japanese macaque in terms of the shift in the functional foot axis. We found that the dorsal interossei are arranged around the third digit; this is true even in the chimpanzee, whose functional axis based on metatarsal length lies on the second digit. This suggests that the change in the arrangement of the interosseous muscles phylogenetically lagged behind the shift of the osteological axis. Our results also indicate that the dorsal interossei are composite muscles consisting of the deep short flexors and the intermetatarsal abductors. We postulate that changes in the contributions of these 2 components to the formation of dorsal interossei likely occurred in the hominin lineage, resulting in the medial shift of the myological axis. The medial shift of the functional foot axis may have started with the elongation of the second metatarsal in the hominoid ancestorsĄÇ lineage, and was completed on the rearrangement of the interosseous muscles.

International Journal of Primatology 31(2): 239-261 (2010)

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