Soleus Muscle

Soleus Muscle

In humans and some other mammals, the soleus is a powerful muscle in the back part of the lower leg (the calf). It runs from just below the knee to the heel, and is involved in standing and walking. It is closely connected to the gastrocnemius muscle and some anatomists consider them to be a single muscle, the triceps surae. Its name is derived from the Latin word, "solea," meaning "sandal."

The soleus is located in the superficial posterior compartment of the leg. Not all mammals have a soleus muscle; one familiar species that lacks the soleus is the dog. Soleus is vestigial in the horse.

The soleus exhibits significant morphological differences across species. It is unipennate in many species. In some animals, such as the rabbit, it is fused for much of its length with the gastrocnemius muscle. In the human, soleus is a complex multi-pennate muscle, usually having a separate (posterior) aponeurosis from the gastrocnemius muscle. A majority of soleus muscle fibers originate from each side of the anterior aponeurosis, attached to the tibia and fibula. Other fibers originate from the posterior (back) surfaces of the head of the fibula and its upper quarter, as well as the middle third of the medial border of the tibia.

The fibers originating from the anterior surface of the anterior aponeurosis insert onto the median septum and the fibers originating from the posterior surface of the anterior aponeurosis insert onto the posterior aponeurosis. The posterior aponeurosis and median septum join in the lower quarter of the muscle and then join with the anterior aponeuroses of the gastrocnemius muscles to form the calcaneal tendon or Achilles tendon and inserts onto the posterior surface of the calcaneus, or heel bone.

In contrast to some animals, the human soleus and gastrocnemius muscles are relatively separate, such that shear can be detected between the soleus and gastrocnemius aponeuroses.