Lateral DeltoidAcromial Part of Deltoid

shouldersPars acromialis musculi deltoidei

The lateral deltoid is the middle section of the shoulder, running from the bony point at the top of the shoulder down to the arm. It lifts the arm out to the side, and it is the part that makes shoulders look wide.

Lateral Deltoid highlighted on an anatomical figure, from the front.
Where it sits, from the front.

The lateral deltoid sits directly over the joint, running from the acromion — the bony ridge you can feel at the very top of the shoulder — straight down the outside of the upper arm. Because the fibres run vertically along the outside of the joint, they lift the arm out sideways and do almost nothing else.

It is built for that job specifically. Unlike the front and rear sections, whose fibres run in long parallel lines, the lateral section is multipennate: short fibres angled into several internal tendons. That packs many more fibres into the same volume and produces much more force over a shorter distance, which is what holding an arm out horizontally demands.

It is also the part of the shoulder that decides how wide someone looks from the front. Shoulder width above the ribcage is essentially skeletal, but the visible outline is this muscle, and it is the one that changes with training.

Training notes

Lateral raises are the movement, and they are the exercise most commonly done with too much weight. The failure is easy to spot: if the shoulders rise toward the ears at the top, the upper traps have taken over and the side delt has been skipped. A weight light enough that the arms do the lifting is doing more than a heavier one that the body swings.

Cable lateral raises are worth knowing about because of where the resistance is hardest. With a dumbbell the movement is hardest at the top and almost free at the bottom; with a cable pulling from the floor across the body it stays loaded through the whole range, including the bottom where the muscle is longest.

Stopping at shoulder height is the usual advice and it is roughly right, but for a reason worth knowing: above that point the shoulder blade has to rotate to let the arm keep going, so the traps and serratus take over the movement. Going higher is not wrong, it is just no longer a lateral raise.

The side deltoid is one of the few muscles where high repetition work is clearly better than heavy work — it is small, it has a short range, and the load a raise can hold is limited by everything else in the chain long before it is limited by the muscle.

Anatomy

Origin
  • Lateral border of the acromion
Insertion
  • Deltoid tuberosity of the humerus
Actions
  • Abducts the arm out to the side — the movement it dominates
  • Helps hold the shoulder joint together while the arm is loaded
Innervation
Axillary nerve (C5–C6)

Exercises

Antagonists

Synergists