The soleus is a powerful, flat muscle located in the posterior (back) compartment of the lower leg. It sits deep beneath the larger and more visible gastrocnemius muscle. Together, they form the triceps surae, which is responsible for plantarflexion of the ankle — the movement of pointing the foot downward.
Unlike muscles designed for explosive movement, the soleus is built for endurance. It works continuously during standing, walking, and maintaining posture. Despite comprising only about 1% of total body mass, research shows it becomes the dominant energy-consuming muscle during low-intensity seated activity.
The soleus originates from the posterior surfaces of the tibia and fibula (the two bones of the lower leg) and joins the calcaneal tendon (Achilles tendon), which inserts into the heel bone (calcaneus). It lies in close proximity to the deep veins of the calf.
The soleus is called the second heart because of its role in supporting venous return. When it contracts, it helps pump blood upward from the lower legs against gravity. This function is critical for circulation, especially during standing and walking. The term appears in published scientific literature.
Read more: Why is the soleus called the second heart? →
When you sit with your knees bent at approximately 90 degrees, the soleus becomes substantially less active compared with standing or walking. This is relevant for people who spend 8 to 10 hours seated each day — the muscle designed for endurance work spends most of the day inactive. Learn about activating the soleus while sitting →
What happens when the soleus stops moving? →
Educational purpose only. This glossary entry is for informational purposes and is not medical advice.