​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."

Function:

The action of the calf muscles, including the soleus, is plantarflexion of the foot (that is, they increase the angle between the foot and the leg). They are powerful muscles and are vital in walking, running, and dancing. The soleus specifically plays an important role in maintaining standing posture; if not for its constant pull, the body would fall forward.

Also, in upright posture, it is responsible for pumping venous blood back into the heart from the periphery, and is often called the skeletal-muscle pump, peripheral heart or the sural (tricipital) pump.

Soleus muscles have a higher proportion of slow muscle fibers than many other muscles. In some animals, such as the guinea pig and cat, soleus consists of 100% slow muscle fibers.

Human soleus fiber composition is quite variable, containing between 60 and 100% slow fibers.