NETA Personal Trainer Practice Exam

The posterior deltoid is primarily responsible for shoulder horizontal abduction. This muscle originates from the spine of the scapula and inserts on the deltoid tuberosity of the humerus, allowing it to play a key role in moving the arm away from the midline of the body when it is in a horizontal position.

During shoulder horizontal abduction, the arm is lifted sideways from a position in front of the body, effectively moving it backward and outward. This movement is important in various athletic and functional tasks, such as pulling or reaching actions.

The other actions listed do not accurately relate to the main function of the posterior deltoid. Shoulder extension involves moving the arm backward from a position at the side of the body, which also involves other muscles like the latissimus dorsi and teres major. Shoulder horizontal adduction refers to moving the arm across the body in a forward direction, while shoulder flexion involves raising the arm in front of the body, both of which are movements primarily facilitated by the anterior deltoid and other muscle groups, rather than the posterior deltoid.