During a strenuous game of tennis a 55 year old woman complained of severe shoulder pain that forced her to quit the game. During physical examination it was found that she could not initiate abduction of her arm, but if her arm was elevated to 45 degrees from the veical (at her side) position, she had no trouble fully abducting it. Injury to which muscle was responsible?

Supraspinatus is responsible for initiating abduction of the arm, while deltoid is responsible for continuing abduction of the arm past the first 15 or 20 degrees. Since this patient can abduct her arm when it is lifted to 45 degrees, deltoid seems to be intact. But, she can't initiate the motion, so you know that she has probably injured supraspinatus. Infraspinatus rotates the arm laterally, and teres major rotates the arm medially. Trapezius elevates and depresses the scapula.A problem with this muscle is most evident if a patient has trouble raising the acromion of her shoulder.