The Achilles tendon or heel cord, also known as the calcaneal tendon (), is a tendon of the back of the leg, and the thickest in the human body. It serves to attach the plantaris, gastrocnemius (calf) and soleus muscles to the calcaneus (heel) bone. These muscles, acting via the tendon, cause plantar flexion of the foot at the ankle, and flexion at the knee. The tendon can rupture and become inflamed. “Achilles heel”, referring to a vulnerability, relates to the mythical story of Achilles, who was slain during the Trojan war by a poisoned arrow to his heel.