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Portrait of Trapezius

Anatomy · Muscles

Trapezius

Named for its diamond or trapezoid shape

Back and neck · superficial muscle of the shoulder girdle

A large, flat, triangular muscle spread over the back of the neck and the upper and middle back. Left and right muscles together make a diamond or trapezoid, which gives it its name. It lifts, retracts and rotates the shoulder blade, so it shrugs the shoulders and helps to raise the arm above the horizontal.

How to spot them

  • ▸A diamond-shaped sheet, apex at the skull and base at the twelfth thoracic vertebra, when both sides are seen together
  • ▸It forms the slope from the neck to the tip of the shoulder
  • ▸Attaches to the lateral third of the clavicle, the acromion and the spine of the scapula
  • ▸Supplied by the accessory nerve (CN XI), which crosses the posterior triangle of the neck

What to know

A diamond-shaped muscle of the back when both sides are viewed together, from the base of the skull to the last thoracic vertebra

Trapezius — trapezoid, from the Greek for a small table.

Its upper fibres lift the shoulder blade as in a shrug, its middle fibres pull it toward the spine and its lower fibres pull it down

Trapezius — three parts, three actions on the scapula.

Damage to the nerve that runs across the posterior triangle of the neck, for instance in a lymph node biopsy, leaves the shoulder drooping and weak to shrug

Trapezius — the spinal accessory nerve, cranial nerve XI.

It attaches along the spine of the scapula, the acromion and the outer third of the clavicle

Trapezius — its insertions on the shoulder girdle.

An older Latin name compares it to a monk's hood

Trapezius — sometimes called the cucullaris, or cowl muscle.

One side of a small gap in the muscles of the back where the chest wall is uncovered and lung sounds are easy to hear, with the lower edge of the scapula and the broad muscle of the lower back

Trapezius — the triangle of auscultation.

Tension in this muscle produces the familiar ache in the upper back and shoulders after long hours at a desk

Trapezius — often the site of trigger points and neck-shoulder pain.

Key ideas

  • It is unusual among back muscles in being supplied by a cranial nerve rather than by spinal nerves alone, because it shares an embryological origin with sternocleidomastoid; both are tested through the accessory nerve.
  • The upper and lower fibres act as a team, rotating the scapula so that the socket faces upwards, which is what allows the arm to be raised above 90 degrees, with serratus anterior.

Quick-fire facts

  • •Origin: the medial third of the superior nuchal line, the external occipital protuberance, the nuchal ligament and the spinous processes of C7 to T12
  • •Insertion: the lateral third of the clavicle, the acromion and the spine of the scapula
  • •Action: upper fibres elevate the scapula, middle fibres retract it and lower fibres depress it; upper and lower fibres together rotate it to raise the arm
  • •Nerve supply: the accessory nerve (CN XI) for movement, with C3 and C4 carrying position sense
  • •Blood supply: the transverse cervical artery, with contributions from the occipital and posterior intercostal arteries
  • •Injury to the accessory nerve, a risk during operations in the posterior triangle of the neck, causes a drooping shoulder and difficulty raising the arm above the horizontal
  • •The triangle of auscultation is bounded by trapezius, latissimus dorsi and the medial border of the scapula
  • •The two muscles together form a trapezoid, from the Greek trapezion, a little table
  • •Its lateral edge forms the posterior boundary of the posterior triangle of the neck, with sternocleidomastoid at the front

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