A 53-year-old man comes to the office with a dull ache in his right shoulder. The pain is worse with movement and often interferes with his sleep. The patient cannot recall any traumatic events prior to the start of the pain but did experience increased discomfort after helping his daughter move into her college dorm room a week ago. His past medical history includes dyslipidemia and hypertension, for which he takes the appropriate medications. Physical examination reveals localized tenderness just below the acromion. The physician asks the patient to abduct his arms 90 degrees to the side and flex them 30 degrees forward with his thumbs pointing to the floor. She then applies downward force to his arms. This maneuver elicits pain in the patient's right shoulder and reveals right-sided weakness as compared to the left side. A tendon of which of the following muscles is most likely injured in this patient? A) deltoid B) biceps brachii C) brachioradialis D) supraspinatus E) serratus anterior The rotator cuff consists of the tendons of the supraspinatus, infraspinatus, teres minor, and subscapularis (SITS) and contributes to the stability and motion of the glenohumeral joint. During abduction of the humerus, the supraspinatus initiates movement through the first 10-15 degrees; subsequently, the deltoid provides the primary abductive force while the supraspinatus provides stability to the joint. Of all the rotator cuff structures, the supraspinatus tendon is most commonly affected in rotator cuff syndrome. This tendon is vulnerable to chronic repeated trauma from impingement between the head of the humerus and the acromion during abduction. Inflammation and fibrosis can worsen the problem by increasing friction between the head of the humerus and the acromion, as well as causing inflammation of the subacromial bursa. On examination, the action of the supraspinatus can be isolated with the "empty-can"