soleus

The soleus is a broad, thick muscle of the posterior leg. It lies deep to the gastrocnemius and, together with the gastrocnemius and plantaris, forms the triceps surae. Its primary role is plantar flexion of the foot at the ankle and maintaining upright posture during standing and locomotion.

Anatomically, the soleus originates from the posterior surfaces of the tibia (including the region around the soleal line) and the proximal posterior surface of the fibula, as well as the interosseous membrane and a tendinous arch between the two bones. Its fibers converge to form the calcaneal (Achilles) tendon, which inserts into the calcaneus. The muscle is innervated by the tibial nerve (L5–S2), and its blood supply comes from branches of the posterior tibial artery and perforating branches from the peroneal artery.

Functionally, the soleus comprises a high proportion of slow-twitch muscle fibers, enabling endurance and sustained plantar flexion. It acts in concert with the gastrocnemius to plantarflex the foot, but unlike the gastrocnemius it does not cross the knee, so it does not contribute to knee flexion.

Clinical notes include the fact that tightness or strain of the soleus can limit ankle dorsiflexion and affect gait. It can be involved in calf strains or in conditions such as Achilles tendinopathy, and it plays a key role in the storage and release of elastic energy through the Achilles tendon during running and jumping.