The soleus is the plantar flexor muscle of the ankle. It is capable of exerting powerful forces onto the ankle joint. It is located on the back of the lower leg and originates at the posterior (rear) aspect of the fibular head and the medial border of the tibial shaft.

The soleus muscle forms the Achilles tendon when it inserts into the gastrocnemius aponeurosis. The tibial nerves S1 and S2 innervate it; arterial sources include the sural, peronial, and posterior tibial arteries.

The soleus muscle is primarily used for pushing off the ground while walking. It may be exercised through calf raises while standing up or sitting down. The soleus is vital to everyday activities such as dancing, running, and walking. The soleus muscle helps to maintain posture by preventing the body from falling forward.

The soleus is also part of the skeletal-muscle pump, which is a collection of muscles that help the heart circulate blood. Veins within the muscles become compressed and decompressed as the muscles surrounding them contract and relax. This aids in venous return of blood to the heart.