Tendinitis is inflammation or irritation of a tendon — any one of the thick fibrous cords that attaches muscle to bone. The condition causes pain and tenderness just outside a joint. While tendinitis can occur in any of your body’s tendons, it’s most common around your shoulders, elbows, wrists, knees, and heels. If tendinitis is severe and leads to the rupture of a tendon, you may need surgical repair.