The rear deltoid is one of three divisions of the deltoid muscle in the shoulder. Forming the rounded top of the shoulder, the deltoid is a superficial muscle, lying close to the skin. If it could be removed and laid flat, it would be triangular in shape, hence its name. The rear or posterior deltoid is the section of the muscle found on the back of the shoulder.
Along with the anterior and middle deltoid, the rear deltoid inserts via a common tendon partway down the shaft of the humerus to the deltoid tuberosity, a V-shaped area on the lateral or outer side of the bone. It has a separate origin, however, arising from the posterior border of the spine of the scapula, a bony ridge running horizontally and slightly downward across the back of the shoulder blade. When a person is standing with the arms at the sides and palms facing forward, the rear deltoid runs almost completely horizontally from the upper shoulder blade across the back of the shoulder and can easily be felt above the back of the arm where it meets the shoulder.
Though it is part of one continuous muscle, the rear deltoid performs a slightly different function than the other two divisions. All are involved in abduction of the shoulder, or lifting of the arm laterally away from the body. The rear deltoid, however, is responsible for what is known as horizontal or transverse abduction of the shoulder, in which an arm raised in front of the body to shoulder height is brought horizontally around to the side. It also extends the shoulder and is the primary muscle involved in shoulder hyperextension. This means that it brings an arm that has been raised in front of the body down to one’s side; hyperextension is the further extension of the shoulder behind the body.
Typically the weakest and most underutilized portion of the deltoid, the rear deltoid can be strengthened via any of several exercises. One such exercise is the reverse fly, which can be performed using dumbbells or a machine specific to this exercise. To perform the dumbbell reverse fly, one should either lie face down on a bench or stand bent forward at the hips so that the back is flat and the torso is parallel to the floor. With a dumbbell in each hand and arms hanging down toward the floor, the arms should be brought upward and outward to shoulder height, keeping the elbows straight, and then returned to the start position. It is recommended to perform two to four sets and no more than 12 repetitions for increases in strength.