Multivariate analysis of variations in intrinsic foot musculature among hominoids

Abstract

Comparative analysis of the foot muscle architecture among extant great apes is important for understanding the evolution of the human foot and, hence, human habitual bipedal walking. However, to our knowledge, there is no previous report of a quantitative comparison of hominoid intrinsic foot muscle dimensions. In the present study, we quantitatively compared muscle dimensions of the hominoid foot by means of multivariate analysis. The foot muscle mass and physiological cross-sectional area (PCSA) of five chimpanzees, one bonobo, two gorillas, and six orangutans were obtained by our own dissections, and those of humans were taken from published accounts. The muscle mass and PCSA were respectively divided by the total mass and total PCSA of the intrinsic muscles of the entire foot for normalization. Variations in muscle architecture among human and extant great apes were quantified based on principal component analysis. Our results demonstrated that the muscle architecture of the orangutan was the most distinctive, having a larger first dorsal interosseous muscle and smaller abductor hallucis brevis muscle. On the other hand, the gorilla was found to be unique in having a larger abductor digiti minimi muscle. Humans were distinguished from extant great apes by a larger quadratus plantae muscle. The chimpanzee and the bonobo appeared to have very similar muscle architecture, with an intermediate position between the human and the orangutan. These differences (or similarities) in architecture of the intrinsic foot muscles among humans and great apes correspond well to the differences in phylogeny, positional behavior, and locomotion.

A Three-Dimensional Musculoskeletal Model of the Western Lowland Gorilla Foot: Examining Muscle Torques and Function

“…These results correspond to that of Oishi [2009] which show that D5 has increased grasping capability in western lowland gorillas. Furthermore, Oishi et al [2018] have shown that abductor digiti minimi in gorilla are better developed, to some extent due to an additional muscle belly, which was found in this study too, compared to other great apes. Secondly, the peak summed flexor torques from proximal interphalangeal 4 (approx.…”

“…The only published data on the functions of the interosseous muscles and lumbricals in gorilla feet are those in Ochiltree [1912], Preuschoft [1961], Hirasaki and Oishi [2018] and Oishi et al [2018] reflecting the difficulty of obtaining dissection data from these small muscles. Interosseous muscles are muscles that originate from metatarsals and insert onto proximal phalanges and metatarsophalangeal joints (and the extensor expansion for dorsal interossei) [Diogo et al, 2010].…”

“…Only recently [Hirasaki and Oishi, 2018] have gorilla foot interosseous attachments been reported, but no lumbricals were included. Most studies of other apes suggest that interossei function as abductors or adductors [Straus, 1949;Sokoloff, 1972;Vereecke et al, 2005;Hirasaki and Oishi, 2018;Oishi et al, 2018]. Studies on other apes such as gibbons and bonobos in Vereecke et al [2005] and orang-utans in Rose [1988] have suggested that the dorsal interossei act as flexors because of their relatively plantar position.…”

“…In chimpanzees, it has also been suggested that the interossei's main function are plantar flexors [Kohlbrugge, 1897;Preuschoft, 1970a]. Only two studies [Ochiltree, 1912;Oishi et al, 2018] mention the possibility of the dorsal interossei in gorilla acting as a flexor. However, given that the interossei are found either medial or lateral to joints, it is indeed possible that their function varies between extensor and flexor as the joint angle changes.…”

Rethinking the evolution of the human foot: insights from experimental research

“…These findings suggest that great apes can stiffen their midfoot joints during bipedal walking even without a longitudinal arch, undermining a major assumption of the dichotomy between human and great ape foot function. Based on the amended windlass model, the most likely mechanism behind this stiffening is activation of the foot muscles, whose attachments and sizes are generally similar in great apes and humans (Oishi et al, 2018). Although in vivo electromyography data are currently lacking, we predict that intrinsic foot muscle activity is closely correlated with foot stiffness during walking in great apes.…”