Evolutionary Implications of the Human Soleus Muscle Based on the Comparative Anatomy of Detailed Intramuscular Nerve Distribution Patterns in Primates

Over the course of its evolution, the human soleus muscle has rapidly increased in size and gained a more important role in bipedal locomotion. However, the detailed processes underlying these morphological changes remain uninvestigated. When discussing these morphological changes in muscles, the innervation patterns among primates is an important criterion to consider and compare. In this study, we comprehensively investigated the detailed intramuscular nerve distribution patterns of the soleus muscle in nine extant primate species and provide new evolutionary implications. The human soleus muscle is innervated by two branches of the tibial nerve: the posterior nerve branch innervates the major posterior part of the soleus muscle, and the anterior nerve branch innervates the anterior bipennate part of the soleus muscle. The soleus muscle is innervated by the posterior branch in all species and by the anterior branch in five of the nine primate species. The prevalence and distribution patterns of the anterior branches varies even between closely related species. However, these variations were not associated with the intramuscular distribution patterns of the posterior branches. Therefore, the distribution of the anterior and posterior branches may have evolved independent of each other. In humans, the anterior branch and intramuscular subbranch of the posterior branch—that innervates the muscle fascicles originating from the soleal line on the tibia—are distributed more widely within the soleus muscle than in non-human primates. This rapid increase in size and medial expansion of the soleus muscle over the course of human evolution may be due to the expansion of the two parts of the soleus muscle innervated by these two branches.