Achilles tendonitis causes pain in the back of the leg or above the heel. It is caused by repetitive or intense strain on the Achilles tendon, the band of tissue that connects your calf muscles to your heel bone. This tendon is used when you walk, run, jump or push up on your toes. Although the Achilles tendon can withstand great stresses from running and jumping, it is also prone to tendonitis, a condition associated with overuse and degeneration.

Overuse, or overload, occurs when there is a sudden increase in the amount or intensity of exercise activity, or when musculoskeletal imbalances place added stress on the Achilles tendon with regular activity, such as when the calf muscles are tight. Degeneration also occurs when the structure of the Achilles tendon weakens with age, which can make it more susceptible to injury.