The shoulder muscles can be divided into the anterior or front muscles, acromion or top muscles, and the posterior or back muscles.
The front muscles include the serratus anterior, subclavius, and pectoralis minor. The serratus anterior is responsible for rotating the shoulder and pulling it forward. The subclavius moves the shoulder downward and forward. The pectoralis minor also help to pull the shoulder forward and downward.
The deltoid muscles form the shape of the shoulder. Although they are consider on top of the shoulder, they extend to the front and back of the shoulder. They are responsible for moving the shoulder toward the front or back, and away to the side of the body.
The muscles at the back of the shoulder include the levator scapulae, trapezius, rhomboid major, and rhomboid minor. The lavator scapulae helps move the shoulder up and down. The trapezius muscles support up and down movement, as well as upward and forward rotation of the shoulder. The rhomboid major and minor work together to provide downward rotation of the shoulder, and forward movement of the scapula.
The sternocleidomastoid, although primarily a neck muscle, also play a role in helping respiration by raising the sternoclavicular joint.