Posterior DeltoidScapular Spinal Part of Deltoid

shouldersPars spinalis scapularis musculi deltoidei

The posterior deltoid is the rear third of the shoulder, running from the ridge of the shoulder blade to the upper arm. It pulls the arm backward, and it is the most commonly under-trained part of the shoulder.

Posterior Deltoid highlighted on an anatomical figure, from behind.
Where it sits, from behind.

The rear deltoid runs from the horizontal ridge on the back of the shoulder blade, forward and downward to the same point on the arm as the other two sections. Because it approaches from behind, it does the opposite of what the front section does: it pulls the arm back rather than forward.

Its two useful actions are extension — drawing the arm behind you — and horizontal abduction, which is sweeping the arm out to the side at shoulder height. Stand with your arms out in front at chest height and pull them apart, and the rear delts are what is working. It also rotates the arm outward, which is why it is often trained alongside the rotator cuff.

It has a genuine problem of visibility. You cannot see it in a mirror while training it, it is small, and the movements that train it use weights that look trivially light next to everything else in a shoulder session. All three make it easy to skip.

It is easily confused with the middle trapezius and the rhomboids, which sit right next to it and are trained by similar-looking movements. The distinction is what moves: the rear delt moves the arm at the shoulder, while the traps and rhomboids move the shoulder blade. A movement that finishes with the shoulder blades squeezed together has trained those; one that finishes with the arms out wide and the shoulder blades relatively still has trained this.

Training notes

Reverse flyes, face pulls, rear delt rows and wide-elbow rows all work, and what they share is the elbow travelling backward at roughly shoulder height rather than tucked at the ribs. Elbow height is the single variable that decides whether a row trains the rear delt or the lat.

Weight is almost always the problem. This is a small muscle at the end of a long lever, and the load it can actually handle is much lighter than most people use. Once the torso starts swinging or the elbows bend to shorten the lever, the movement has become a row.

It is the most reliably under-trained part of the shoulder because nothing outside a gym does what it does, while a great deal of ordinary life and most of an upper-body program does the opposite. If one thing gets added to a shoulder routine, this is it.

Since it also externally rotates the arm, rear delt work and rotator cuff work sit naturally together at the end of a session, at similar light loads, without either competing with the other.

Anatomy

Origin
  • Lower border of the spine of the scapula
Insertion
  • Deltoid tuberosity of the humerus
Actions
  • Extends the arm, drawing it backward behind the body
  • Horizontally abducts the arm out to the side and back
  • Externally rotates the humerus
  • Assists abduction of the arm
Innervation
Axillary nerve (C5–C6)

Exercises

Antagonists

Synergists