A 19-year-old man is brought to the emergency department after dislocating his shoulder while playing football. Following treatment of the dislocation, he cannot initiate abduction of his arm. An MRI of the shoulder shows a torn muscle. Which muscle was most likely damaged by the injury?

The rotator cuff muscles are common sites of damage during shoulder injuries. These muscles include the supraspinatus, infraspinatus, teres minor, and subscapularis (SITS). Initiation of abduction of the humerus (the first 15deg) is performed by the supraspinatus, followed by the deltoid from 15deg to 90deg. Above the horizontal, the humerus is abducted by the trapezius and serratus anterior. The teres major and the pectoralis major are responsible for medial rotation and adduction of the humerus. These muscles are therefore not involved in abduction at the glenohumeral joint.