The structure indicated is the calcaneal tendon (Achilles’ tendon).

The calcaneal tendon is formed from the convergence of the three muscles of the superficial compartment of the posterior leg to form a tendon that inserts onto the calcaneus. The posterior compartment of the leg consists of a superficial group of muscles and a deep group of muscles.

The superficial group consists of the following leg muscles:

• Gastrocnemius
• Plantaris
• Soleus

The deep group consists of the following leg muscles:

• Popliteus
• Flexor hallucis longus
• Flexor digitorum longus
• Tibialis posterior

The calcaneal tendon is the thickest tendon in the human body. Contraction of the muscles attaching via this tendon results in plantarflexion of the foot at the ankle joint. The calcaneal tendon gets the name “Achilles’ tendon” from Greek mythology whereby the Greek hero Achilles was killed by Paris of Troy in the Trojan war by a poisoned arrow to the heel.

Learn more about the anatomy of the leg muscles in this tutorial.