Comparison of the soleus and plantaris muscles in humans and other primates: Macroscopic neuromuscular anatomy and evolutionary significance

Abstract

In humans, the soleus is more developed compared to other primates and has a unique architecture composed of anterior bipennate and posterior unipennate parts, which are innervated by different nerve branches. The anterior part of the human soleus was proposed to be important for bipedalism, however, the phylogenetic process resulting in its acquisition remains unclear.Providing insights into this process, the anterior part of the soleus was suggested to be closely related to the plantaris based on the branching pattern of their nerve fascicles. To reveal the phylogeny of the soleus and plantaris in primates, the innervation patterns of the posterior crural muscles were compared among a wide range of species. From their branching pattern, posterior crural muscles could be classified into superficial and deep muscle groups. The anterior part of the soleus and plantaris both belonged to the deep muscle group. In all the examined specimens of ring-tailed lemurs and chimpanzees, as well as in one out of two specimens of siamang, the nerve branches corresponding to those innervating the anterior part of the human soleus were found. The muscular branches innervating the anterior part of the soleus and plantaris formed a common trunk or were connected in all the specimens.These results indicate that the anterior part of the soleus is closely related to the plantaris across different species of primates. In turn, this suggests that the anterior part of the soleus is maintained among primates, and especially in humans, where it develops as the characteristic bipennate structure.