Your Achilles tendon may be the reason humans can run at all
The thickest tendon in your body takes loads of 7.7 times your body weight with every running stride and stores that energy like a spring. Computer models suggest it lifts top running speed by more than 80 per cent. Chimpanzees and gorillas have short or missing versions, which may explain why they cannot run far.
The cord runs about 15 cm down the back of the lower leg, tying the calf muscles, the gastrocnemius and soleus, to the heel bone. When they contract, it points the foot downward. Its fibres twist about 90 degrees on the way down, so strands from each muscle land on different parts of the heel. Even walking loads it with 3.9 times body weight, yet its blood supply is poor.
Hippocrates called it the great tendon, and later anatomists the string of Hippocrates. The name we use appeared in 1693, when the Flemish anatomist Philip Verheyen wrote that it was commonly known as the cord of Achilles. He was recalling the myth in which Thetis dipped the baby Achilles in the River Styx, holding him by the heel, which stayed dry and became his fatal weak point. The name also nods to how crippling an injury here can be. Because such eponyms describe nothing, anatomists also call it the calcaneal tendon, after the Latin for heel.
It can fail in several ways. A rupture strikes about 1 in 10,000 people a year, most often men between their 30s and 50s, and can announce itself with sudden sharp heel pain and an audible snap. Common triggers include forcefully flexing the foot upward while the calf is tensed, direct blows and long-running tendinitis, and certain antibiotics and steroids raise the risk. The surgeon Ambroise Paré described the first closed rupture in the 16th century. In people with inherited cholesterol disorders, fatty deposits called xanthomas often collect in the tendon.
Doctors also use it as a window on the nervous system. Tapping it with a reflex hammer checks the S1 and S2 spinal nerves, and the normal response is a downward flick of the foot. Vibrating the tendon in someone standing with eyes closed tricks calf sensors into reporting a forward lean, and the body sways backward to compensate.
Source: Achilles tendon
