In humans and some other mammals, the soleus is a powerful muscle in the back part of the lower leg (the calf).

Most soleus muscle fibers originate from each side of the anterior aponeurosis, attached to the tibia and fibula.

[1][2] 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.

[1][2] 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.

The soleus plays an important role in maintaining standing posture; if not for its constant pull, the body would fall forward.

During regular movement (i.e., walking) the soleus is the primary muscle utilized for plantar flexion due to the slow-twitch fibers resisting fatigue.

[9] The soleus is innervated by the tibial nerve (L4, L5, S1, S2) Due to the thick fascia covering the leg muscles, they are prone to compartment syndrome.