Simply defined, tendinitis is inflammation of a tendon. Inflammation is the body's natural response to injury or disease, and often causes swelling, pain, or irritation. There are two types of Achilles tendinitis, based upon which part of the tendon is inflamed.

Non-Insertional Achilles Tendinosis

In noninsertional Achilles tendinitis, fibers in the middle portion of the tendon have begun to break down with tiny tears (degenerate), swell, and thicken.

Tendinitis of the middle portion of the tendon more commonly affects younger, active people.

Insertional Achilles Tendonitis

Insertional Achilles Tendinitis involves the lower portion of the heel, where the tendon attaches (inserts) to the heel bone. In both noninsertional and insertional Achilles tendinitis, damaged tendon fibers may also calcify (harden). Bone spurs (extra bone growth) often form with insertional Achilles tendinitis.

Tendinitis that affects the insertion of the tendon can occur at any time, even in patients who are not active.