Achilles tendinitis is an overuse injury of the Achilles tendon, the band of tissue that connects calf muscles at the back of the lower leg to your heel bone. Achilles tendinitis most commonly occurs in runners who have suddenly increased the intensity or duration of their runs.They may be caused by strain, overuse, injury, or too much exercise. Tendonitis may also be related to a disease such as diabetes, rheumatoid arthritis, or infection.