There are many different types of Achilles tendon injuries. Achilles tendonitis, one of the most common Achilles tendon injuries, is an inflammation of the tendon due to repetitive strain and overuse. Tendonitis causes pain and stiffness at the back of the heel. Achilles tendonosis refers to a gradual thickening and weakening of the Achilles tendon due to overuse or aging. More serious injuries to the Achilles tendon are tears or full ruptures. An Achilles tendon tear can be small or large, and may cause symptoms such as pain, swelling, and difficulty moving the affected leg. A rupture occurs when the tendon tears completely, and is accompanied by intense pain and swelling.