The deltoid muscle is a large, triangular muscle that gives the shoulder its rounded contour, acting as the shoulder cap. This muscle group is composed of three distinct heads: the anterior (front), the medial (side), and the posterior (rear) deltoid. While the front and side heads are clearly associated with shoulder function, the posterior deltoid, or “rear delt,” is frequently misclassified due to its location near the upper back. This common confusion stems from its anatomical position and its shared function with several back muscles during pulling movements.
Anatomical Placement of the Deltoid Muscle
The posterior deltoid is definitively classified as a muscle of the shoulder, not a muscle of the back. The entire deltoid muscle group, including the posterior head, originates across the clavicle (collarbone) and the scapula (shoulder blade) and inserts onto a single point on the humerus (upper arm bone) called the deltoid tuberosity. This attachment to the humerus qualifies it as a shoulder muscle, as it moves the arm at the shoulder joint.
The posterior head specifically arises from the spine of the scapula, which is the prominent ridge running across the back of the shoulder blade. Though it lies adjacent to several upper back muscles, such as the trapezius and the rhomboids, its fiber direction and insertion point on the arm bone distinguish it from the true back musculature.
Primary Function in Shoulder Movement
The reason the posterior deltoid is often mistaken for a back muscle relates directly to the movements it performs. The primary actions of the posterior deltoid are shoulder extension and horizontal abduction. Shoulder extension involves drawing the arm backward, and horizontal abduction involves moving the arm away from the midline of the body in the transverse plane, like the reverse of a chest fly.
The posterior deltoid also contributes to the external rotation of the arm at the shoulder joint. This set of functions often overlaps with the actions of the larger upper back muscles, including the rhomboids and the middle and lower trapezius. Because the rear deltoid acts as a synergist, or helper muscle, during many exercises that primarily target the back, trainees often mistakenly group it with the back muscles.
Importance for Joint Stability and Posture
Beyond movement, the posterior deltoid plays a significant role in maintaining the structural integrity and balance of the shoulder joint. It helps stabilize the head of the humerus, the ball of the ball-and-socket shoulder joint, within the glenoid socket of the scapula. This stabilization is important for preventing the humeral head from shifting or dislocating during activities, especially when carrying heavy loads.
A common issue is the overdevelopment of the anterior deltoid and chest muscles relative to the posterior deltoid, particularly among those who focus heavily on pushing exercises like the bench press. This muscular imbalance creates a forward pull on the shoulder joint, which can lead to poor posture characterized by rounded shoulders and an internally rotated arm position. A strong posterior deltoid helps counteract this forward force, pulling the shoulders back into a more neutral and upright position, which improves overall posture. Furthermore, strengthening this muscle can help prevent common shoulder injuries like impingement by ensuring proper tracking and positioning of the humeral head in the socket during movement.
Targeting the Posterior Head in Training
Effective training of the posterior deltoid requires exercises that isolate its primary functions, minimizing the involvement of the much larger surrounding back muscles. Isolation movements that focus on horizontal abduction are the most effective way to target this muscle head specifically.
The face pull is a highly recommended movement, involving pulling a cable or resistance band toward the face with the elbows flared, which emphasizes the rear deltoid and external rotation. Other isolation exercises include the bent-over dumbbell raise, often called the reverse fly, and the reverse pec deck machine fly. It is important to use a lighter weight that allows for a strict focus on contracting the target muscle, rather than relying on momentum or the stronger trapezius and latissimus dorsi muscles.