Which of the following muscles abducts the arm at the glenohumeral joint?

The deltoid muscle abducts the arm. It consists of three sections: anterior, middle and posterior. The middle deltoid is responsible for abducting the humerus. Because of the insertion points and the angle of pull, the deltoid can only abduct the arm to about 90 degrees, and requires the supraspinatus to initiate abduction. Arm elevation higher then 90 degrees comes from upward rotation of the scapula, which is caused by muscles including: upper and lower trapezius, and serratus anterior.