From Google...
The soleus muscle and surrounding structures, from Gray's Anatomy. This is a view of the back of the right leg; most of the gastrocnemius muscle has been removed.
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 solefish whose shape it resembles. The soleus is located in the superficial posterior compartment of the leg. It originates from the posterior (back) surfaces of the head of the fibula and its upper third, as well as the middle third of the internal border of the tibia. Its other end forms a common tendon with the gastrocnemius muscle; this tendon is known as the calcaneal tendon or Achilles tendon and inserts onto the posterior surface of the calcaneus, or heel bone.