In tendinosis, wear and tear is thought to lead to tissue degeneration. A degenerated tendon usually has an abnormal arrangement of collagen fibers.

Instead of inflammatory cells, the body produces a type of cells called fibroblasts. When this happens, the collagen loses its strength. It becomes fragile and can break or be easily injured. Each time the collagen breaks down, the body responds by forming scar tissue in the tendon. Eventually, the tendon becomes thickened from extra scar tissue.

No one really knows exactly what causes tendinosis. Some doctors think that the forearm tendon develops small tears with too much activity. The tears try to heal, but constant strain and overuse keep re-injuring the tendon. After a while, the tendons stop trying to heal. The scar tissue never has a chance to fully heal, leaving the injured areas weakened and painful.