There are two types of Achilles tendinitis, based upon which part of the tendon is inflamed.
Noninsertional Achilles Tendinitis
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 tendinitis
Insertional Achilles tendinitis involves the lower portion of the heel, where the tendon attaches (inserts) to the heel bone.