The soleus muscle

The soleus muscle is a wide flat leg muscle found on the posterior leg. It runs from just below the knee to the heel and lays immediately deep to the gastrocnemius. These two muscles, along with the plantaris muscle, belong to the group of superficial posterior compartment calf muscles. Soleus’ contraction results in strong plantar flexion. It also allows us to maintain an upright posture due to its important role as an antigravity muscle.
Together with the gastrocnemius, they form the three-headed group of muscles referred to as the triceps surae. They both insert on the calcaneus via the calcaneal tendon and act in many basic activities, such as walking, running, and leaping. The size and shape of the triceps surae muscle bellies determine the interindividual differences of human calves’ appearance - from slim to rather robust.
The soleus muscle arises from the soleal line on the dorsal surface of the tibia, medial border of the tibia, head of the fibula, and posterior border of the fibula. Part of the fibers arises from the tendinous arch of the soleus, which spans between the tibia and fibula and arches over the popliteal vessels and tibial nerve.
The soleus muscle runs along the gastrocnemius muscle and together they insert onto the posterior surface of the calcaneus via the calcaneal tendon. The calcaneal tendon, commonly called the Achilles tendon, is the strongest tendon of the human body. It is easily visible and palpable at the heel.
The soleus is innervated by the anterior rami of S1 and S2 spinal nerves, carried by the tibial nerve into the posterior compartment of the leg.