ShouldersDeltoid

shoulders3 musclesMusculus deltoideus

The deltoid is the thick triangular muscle capping your shoulder, and it is what lifts your arm in every direction. Its three parts pull forward, sideways and backward, and they are trained by different movements.

Shoulders: its 3 muscles highlighted on an anatomical figure, from the front — Anterior Deltoid, Lateral Deltoid and Posterior Deltoid.
Where the shoulders sit, from the front.

The deltoid wraps over the top of the shoulder like a shoulder pad, which is exactly what gives shoulders their shape. It attaches along the outer end of the collarbone at the front, over the bony point of the shoulder at the side, and along the ridge of the shoulder blade at the back — then all of those fibres converge downward onto a single rough patch about a third of the way down the upper arm bone.

Because it is anchored across that whole arc, it can pull the arm in any direction depending on which fibres shorten. The front fibres raise the arm forward, the side fibres raise it out sideways, and the rear fibres pull it backward. This is a genuinely unusual arrangement: most muscles pull one way, and the deltoid's three parts routinely oppose each other.

The side fibres are also built differently. They are multipennate — short fibres running at an angle into several internal tendons, like a series of feathers — which trades range of motion for force. That is the arrangement you find in muscles that have to lift a long lever, and holding an arm out horizontally is exactly that problem.

Almost everything the deltoid does depends on the rotator cuff underneath it doing its job at the same time. The deltoid's line of pull runs up the arm bone, so on its own it would drag the ball of the shoulder upward rather than rotate it; the cuff pulls it down into the socket so the arm swings instead. Neither works properly without the other.

Training notes

The three parts need three different movements and this is the single most useful thing to know about training shoulders. Pressing trains the front. Lateral raises train the side. Rear delt flyes, face pulls and wide rows train the back. A shoulder session that is only pressing has trained a third of the muscle.

The front deltoid is the part that needs the least help, because it works on every bench press, incline press, dip and push-up as well as on every overhead press. It is common for someone doing a normal upper-body program to have front deltoids well ahead of the other two parts without ever having trained shoulders directly.

The rear deltoid is the reverse: nothing in daily life and very little in a typical program pulls the arm backward and outward, so it is the most reliably under-trained part of the shoulder. It also responds to light weight and high repetitions better than to load, because it is small and easy to swamp with momentum.

Lateral raises are the movement most often ruined by weight. Once the dumbbell is heavy enough that the upper traps shrug the arm up, the side deltoid has been bypassed. Lighter, slower, and stopping at shoulder height is the version that works.

Anatomy

Origin
  • Anterior border of the lateral third of the clavicle
  • Lateral border of the acromion
  • Lower border of the spine of the scapula
Insertion
  • Deltoid tuberosity, on the lateral surface of the shaft of the humerus
Actions
  • Abducts the arm out to the side, from about fifteen degrees onward
  • Flexes and internally rotates the arm, through the anterior fibres
  • Extends and externally rotates the arm, through the posterior fibres
  • Holds the head of the humerus in place while the arm moves under load
Innervation
Axillary nerve (C5–C6)

Parts of this muscle

Exercises

Antagonists

Synergists