Last week we talked about the Gastrocnemius muscle; the Soleus lays under it.
A majority of Soleus muscle fibers originate from each side of the anterior (front) tibia and fibula. 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. The posterior aponeurosis and median border of the tibia join in the lower quarter of the muscle and then join with the anterior aponeurosis 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 specifically plays an important role in maintaining standing posture; if not for its constant pull, the body would fall forward.
The soleus is the most effective muscle for plantar flexion (pointing the foot) in a bent knee position. This is because the gastrocnemius originates on the femur, so bending the leg limits its effective tension. During walking, the soleus is the primary muscle utilized for plantar flexion due to the red (slow-twitch) muscle fibers resisting fatigue.