As the deltoid muscle abducts the arm, it also elevates the humerus, which butts against the acromion of the scapula and can go no further. turn your palm upward and you are able to complete the range of motion overhead. The infraspinatus and the teres minor function to externally rotate the arm and, along with the supraspinatus, draw the humeral head inward and down, thus clearing the acromion and allowing the arm to abduct to 120 degrees