The rotator cuff is a group of muscles forming the common tendon that helps stabilize your shoulder so bigger muscles, such as the deltoid and the pectoralis major, can move your arm. Rotator-cuff tendinitis – also known as bursitis, impingement syndrome and tendinopathy – involves inflammation of the sub-acromial bursa, which lies between the acromion bone and the rotator-cuff muscles (supraspinatus, infraspinatus, teres minor and subscapularis).