The structure indicated is the soleus muscle of the leg.
The soleus is a muscle of the posterior compartment of the leg.The muscles in the posterior compartment of the leg are separated into a superficial group and a deep group.
The superficial group consists of the following leg muscles:
The deep group consists of the following leg muscles:
- Flexor hallucis longus
- Flexor digitorum longus
- Tibialis posterior
The soleus muscle lies deep to the gastrocnemius and is large and flat in shape running the length of the leg from the knee to the heel, closely adherent to the gastrocnemius muscle, with its fibres fused at several points. The soleus muscle joins the plantaris and gastrocnemius muscles to insert onto the calcaneus via a common tendon – the Achilles tendon (calcaneal tendon).
Origin: Soleal line and middle third of posterior border of tibia, and upper quarter of posterior shaft of fibula
Insertion: Via calcaneal tendon to posterior surface of calcaneus
Action: Plantarflexes foot at ankle joint, flexion of knee
Innervation: Tibial nerve
Learn more about the anatomy of the leg muscles in this video tutorial.