Rotator cuff tendinitis affects the tendons and muscles that help move the shoulder joint. Tendinitis means that these tendons are inflamed or irritated. Rotator cuff tendinitis may also be called impingement syndrome. Tendinitis of the rotator cuff usually occurs over time. It can be the result of keeping the shoulder in one position over a period of time, sleeping on the shoulder every night, or participating in activities that require extending the arm over the head.