Deltoid

What it is

The thick triangular muscle forming the rounded contour of the shoulder — named for the Greek letter delta, which it resembles inverted. It wraps the joint from front, side and back.

Its origin is a continuous arc across the outer clavicle, the acromion and the spine of the scapula, and all of it converges onto a single small tuberosity partway down the outer humerus.

Its three parts are functionally distinct enough that anatomists treat them separately:

• Anterior (clavicular) — in front of the joint
• Middle (acromial) — directly over it
• Posterior (spinal) — behind it

The middle part is multipennate, its fibres arranged around several internal tendons, which is a structural arrangement that favours force over range.

What it does

Depends entirely on which part.

The middle part abducts the arm — lifts it out to the side — and is the prime mover for it past the first stretch of the movement.

The anterior part flexes and internally rotates the shoulder, working with pectoralis major.

The posterior part extends and externally rotates it, working with latissimus dorsi.

So the front and back thirds of one muscle are direct opponents. Which part is doing the work is decided by where your arm is, not by intent.

What loads it

Pressing overhead, raising the arm in any direction against resistance, and holding a load away from the body.

The pike push-up loads it most, because a steep press is close to overhead. The push-up and incline push-up load the front of it.

What is commonly got wrong about it

"Three heads" as three separate muscles to be trained on three separate days. They share an insertion and a nerve, and they overlap heavily in most real movements. The distinction is genuine, but it is a distinction of line of pull, not of independent muscles.

That the first part of lifting your arm sideways is deltoid. Supraspinatus initiates abduction and deltoid takes over. See the supraspinatus record — this is one of the more commonly misstated facts in shoulder anatomy.

"Shoulder width" as a training outcome. The visible width of the shoulders is set largely by the width of the collarbones and the shape of the ribcage. Muscle adds to it; it does not determine it.