Anatomy · Muscles

Deltoid

The rounded cap of the shoulder

Upper limb · shoulder

The deltoid is the thick, triangular muscle that gives the shoulder its rounded contour. It arises from the clavicle, acromion and spine of the scapula and inserts on the humerus, and it is the prime abductor of the arm after the first few degrees. It is supplied by the axillary nerve.

How to spot them

▸Rounded cap over the shoulder joint, thick and triangular, wrapping the top of the humerus
▸Fibres converge from clavicle, acromion and scapular spine to a single tendon halfway down the outer arm
▸Named for its resemblance to the Greek capital delta, an inverted triangle
▸Its front, middle and back parts are separate in action, like three muscles in one

What to know

The thick triangular cap over the top of the humerus, forming the outline of the shoulder

Deltoid.

The muscle whose middle fibres do most of the work in raising the arm sideways from about 15 to 90 degrees

Deltoid — the supraspinatus starts the movement, this takes over.

Its tendon lands on a roughened patch halfway down the outer shaft of the humerus

Deltoid tuberosity of the humerus is its insertion.

A muscle supplied by a nerve that winds round the surgical neck of the humerus, so a fracture there may paralyse it

Deltoid — axillary nerve (C5–C6).

The usual muscle for vaccination injections in the upper arm

Deltoid — thick, superficial and easy to reach.

Origin spans the outer third of the collarbone, the acromion and the spine of the scapula

Deltoid — three origins for three parts.

Key ideas

Anterior fibres flex and medially rotate the arm; middle fibres abduct it; posterior fibres extend and laterally rotate it.
The supraspinatus initiates abduction; the deltoid takes over and carries it to horizontal, and the trapezius and serratus anterior rotate the scapula to take it overhead.

Quick-fire facts

•Origin: lateral third of the clavicle (anterior fibres), acromion (middle fibres) and spine of the scapula (posterior fibres).
•Insertion: deltoid tuberosity on the lateral shaft of the humerus.
•Nerve supply: axillary nerve (C5–C6), which also supplies teres minor.
•The axillary nerve runs round the surgical neck of the humerus, so a fracture or shoulder dislocation can injure it and flatten the shoulder.
•The middle fibres are multipennate, which makes them powerful but short-ranging.
•Named from the Greek letter delta (Δ), whose shape the muscle resembles.
•It is a standard site for intramuscular injection because it is bulky and near the surface.

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Sources

TeachMeAnatomy — Deltoid - attachments, actions, innervation