The shoulder girdle is a complex arrangement of muscles and bones that enables the arm’s wide range of motion, and the deltoid muscle group is central to this system. This prominent muscle, which gives the shoulder its rounded contour, is divided into three distinct sections: the anterior (front), the medial (side), and the posterior (rear) deltoid. The posterior deltoid, or “rear delt,” is frequently overlooked in favor of the more visible front and side heads. However, its strength and function are foundational for balanced shoulder health and movement.
Defining the Posterior Deltoid
The posterior deltoid is a triangular muscle situated on the back of the shoulder, forming the rear third of the larger deltoid muscle complex. Its fibers originate from the spine of the scapula, which is the prominent ridge running across the back of the shoulder blade. From this broad origin point, the muscle fibers converge downward and laterally toward the upper arm bone.
The insertion point for the posterior deltoid, along with the anterior and medial heads, is the deltoid tuberosity, a rough patch located on the lateral side of the humerus. This shared insertion allows the three heads to work together to move the arm, despite having different origins and primary functions. The muscle’s position on the back of the shoulder joint makes it mechanically suited for pulling actions rather than pushing motions.
Key Actions in Movement
The primary biomechanical role of the posterior deltoid involves moving the arm backward relative to the torso. One of its main functions is shoulder extension, which is the action of pulling the arm straight back, as occurs during the end of a rowing motion or a backswing when walking.
The rear delt also performs horizontal abduction, moving the arm away from the midline of the body when the arm is held straight out in front. This action is seen when performing a reverse fly movement, moving the hands out to the sides from a bent-over position.
The posterior fibers contribute to external rotation, turning the upper arm bone outward away from the body’s center. This outward rotation is performed in conjunction with smaller rotator cuff muscles, making the rear delt an important component in complex, multi-directional arm movements.
Role in Posture and Shoulder Stability
A strong posterior deltoid is a major factor in maintaining correct posture and ensuring shoulder health. Modern lifestyles and common training routines often emphasize the muscles on the front of the body, such as the chest and anterior deltoids, which can lead to a muscular imbalance. This imbalance contributes to shoulder internal rotation, characterized by a rounded-shoulder or hunched-forward appearance.
The posterior deltoid directly counteracts this forward pull by exerting force in the opposite direction, promoting a more upright and open posture. By pulling the shoulders back, it helps align the head of the humerus correctly within the shoulder socket.
This proper alignment is important for preventing potential shoulder impingement. Additionally, the rear delt stabilizes the shoulder joint during heavy compound movements, such as a bench press, by keeping the humerus secure and centered in the socket.
Effective Training Movements
Targeting the posterior deltoid effectively requires movements that specifically engage its primary functions of horizontal abduction and extension. Exercises should be performed with a focus on feeling the contraction in the back of the shoulder rather than relying on momentum or recruiting larger back muscles like the trapezius.
The Face Pull involves pulling a cable or resistance band toward the face, combining horizontal abduction with external rotation. The Reverse Dumbbell Fly is typically performed while bent over or lying prone on an incline bench, raising light to moderate weight out to the sides. The Bent-Over Lateral Raise isolates the muscle by focusing purely on moving the arm away from the body in the horizontal plane.
Maintaining a controlled tempo and using a weight that allows for a high number of repetitions, often between 10 to 15, is recommended to maximize the muscle’s activation and growth.