The deltoid is a large, triangular muscle that gives the shoulder its rounded contour. This muscle is separated into three distinct sections, or heads: the anterior (front), the medial or lateral (side), and the posterior (rear) deltoid. While all three heads work together for general arm movement, the posterior deltoid, or “rear delt,” often receives the least attention in training.
Anatomy and Location
The posterior deltoid is the rearmost segment of the shoulder muscle group, spanning the back of the shoulder joint. Its origin point is the lower lip of the posterior border of the spine of the scapula (shoulder blade). From there, its fibers run obliquely forward and laterally across the shoulder.
All three heads of the deltoid converge to share a common insertion point on the humerus, the upper arm bone. The rear deltoid inserts into the deltoid tuberosity, a roughened area located on the middle of the lateral side of the humerus. Because daily activities and common exercises primarily target the chest and anterior deltoids, the posterior head is often weaker and less developed than its frontal counterpart.
Primary Functions of the Posterior Deltoid
The posterior deltoid performs several distinct actions at the shoulder joint. Its primary function is shoulder extension, which involves drawing the arm backward (e.g., pulling a cord). This movement works in tandem with muscles like the latissimus dorsi.
A key role is horizontal abduction (transverse extension), which moves the arm away from the midline of the body while held parallel to the ground. The posterior fibers are also responsible for external rotation, turning the upper arm outward. Additionally, the muscle provides dynamic stability to the glenohumeral joint, helping to keep the head of the humerus securely positioned in the shoulder socket.
Incorporating Rear Deltoid Training
Targeting the posterior deltoid is important for balanced shoulder musculature and joint health. If the anterior deltoids and chest muscles are much stronger than the rear delts, the resulting muscle imbalance can pull the shoulders forward, contributing to poor posture. Strengthening the rear delts helps counteract this pull, acting as a retractor that pulls the shoulders back and down.
To effectively isolate this muscle, focus on movement mechanics rather than lifting heavy weight. Exercises that mimic the muscle’s primary functions, such as horizontal abduction, are effective. Common movements include reverse fly variations, performed bent-over with dumbbells or on a machine. For optimal activation, use a light to moderate weight and concentrate on squeezing the shoulder blades together at the peak of the movement.
The face pull is another recommended exercise that targets the rear deltoid and upper back. This movement involves pulling a cable or resistance band towards the face, engaging the external rotation function of the posterior fibers. During all rear delt exercises, avoid using momentum, which recruits larger muscles like the trapezius or rhomboids instead of the target muscle.
Training the rear delts with higher frequency and moderate volume is often beneficial, as they respond well to consistent stimulus. Including two to three exercises for the posterior deltoid across a weekly training schedule can help ensure they receive adequate attention. By prioritizing controlled movement, proper form, and consistent effort with specific exercises, individuals can strengthen these often-overlooked muscles, improving both appearance and long-term shoulder function.