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Tendon at the back of the lower leg

The Achilles tendon or heel cord, also known as the calcaneal tendon, is a tendon at the back of the lower leg, and is 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 joint, and (except the soleus) flexion at the knee. Abnormalities of the Achilles tendon include inflammation (Achilles tendinitis), degeneration, rupture, and becoming embedded with cholesterol deposits (xanthomas). The Achilles tendon was named in 1693 after the Greek hero Achilles.

The oldest known written record of the tendon being named for Achilles is in 1693 by the Flemish/Dutch anatomist Philip Verheyen. In his widely used text Corporis Humani Anatomia he described the tendon's location and said that it was commonly called "the cord of Achilles." The tendon has been described as early as the time of Hippocrates, who described it as the "tendo magnus" (Latin for "great tendon") and by subsequent anatomists prior to Verheyen as "chorda Hippocratis" (Latin for "Hippocrates' string"). The tendon begins near the middle of the calf, and receives muscle fibers on its inner surface, particularly from the soleus muscle, almost to its lower end. Gradually thinning below, it inserts into the middle part of the back of the calcaneus bone. The tendon spreads out somewhat at its lower end so that its narrowest part is about above its insertion. The tendon is the thickest tendon in the human body. It can receive a load stress 3.9 times body weight during walking and 7.7 times body weight when running. The blood supply to the Achilles tendon is poor, and mostly via a recurrent branch of the posterior tibial artery, and some through arterial branches passing through surrounding muscles. Because the fibres of the tendon spiral about 90 degrees, fibres from the gastrocnemius tend to attach to the outer part of the bone, whereas fibres from the soleus tend to attach closer to the midline. Vibration of the tendon causes movement backwards and the illusion of a forward body tilt in standing subjects. This is because vibrations stimulate muscle spindles in the calf muscles. The muscle spindles alert the brain that the body is moving forward, so the central nervous system compensates by moving the body backwards.

Inflammation of the Achilles tendon is called Achilles tendinitis. Achilles tendinosis is the soreness or stiffness of the tendon, particularly worse when exercising, and generally due to overuse. The most common symptoms are sharp achy pain and swelling around the affected tendon. The pain is typically worse at the start of exercise and decreases thereafter. Other risk factors include trauma, a lifestyle that includes little exercise, high heel shoes, rheumatoid arthritis, and medications of the fluoroquinolone or steroid class. Treatment typically involves rest, ice, non steroidal antiinflammatory agents (NSAIDs), and physical therapy.

Achilles tendon rupture is when the Achilles tendon breaks. A snapping sound may be heard as the tendon breaks and walking becomes difficult. Rupture typically occurs as a result of a sudden bending up of the foot when the calf muscle is engaged, direct trauma, or long standing tendonitis. Males are more commonly affected than females. The risk of re rupture is about 25% with casting.

Tendon xanthomas are cholesterol deposits that commonly develop in the Achilles tendon of people with lipid metabolism disorders such as familial hypercholesterolemia.

The Achilles tendon is short or absent in great apes, but long in arboreal gibbons and humans. It provides elastic energy storage in hopping, walking, and running. Computer models suggest this energy storage Achilles tendon increases top running speed by >80% and reduces running costs by more than three quarters. It has been suggested that the "absence of a well developed Achilles tendon in the nonhuman African apes would preclude them from effective running, both at high speeds and over extended distances."