CorpusHuman Anatomy
MuscularMuscle

Deltoid

Musculus deltoideus

Also known as: delts, deltoid muscle

A thick, triangular muscle that caps the shoulder and gives it its rounded contour. Its three parts abduct (15–90°), flex and extend the arm.

Show on body map Muscles — Posterior View

Overview

Deltoid arises from a U-shaped line that mirrors the insertion of trapezius: the lateral third of the clavicle, the acromion and the spine of the scapula. It converges on the deltoid tuberosity halfway down the lateral humerus. It has three parts.

- Clavicular (anterior) part: flexes and medially rotates the arm.
- Acromial (middle) part: multipennate and very powerful. It is the main abductor from ≈15° to 90°.
- Spinal (posterior) part: extends and laterally rotates the arm.

At the very start of abduction, deltoid's line of pull is almost vertical, so it would simply drag the humeral head upward. Supraspinatus initiates the first ≈15° and the rotator cuff holds the head in the glenoid. Above ≈90°, scapular rotation by trapezius and serratus anterior completes elevation. The overall ratio of glenohumeral to scapulothoracic movement is ≈2 : 1 (the scapulohumeral rhythm).

Deltoid is supplied by the axillary nerve (C5–C6), which winds around the surgical neck of the humerus with the posterior circumflex humeral artery, about 5–7 cm below the acromion.

Location

Shoulder, covering the glenohumeral joint anteriorly, laterally and posteriorly.

Parts & subdivisions

  1. Clavicular (anterior) part
    Lateral third of the clavicle. Flexion and medial rotation.
  2. Acromial (middle) part
    Acromion. Multipennate. Abduction 15–90°.
  3. Spinal (posterior) part
    Spine of the scapula. Extension and lateral rotation.

Anatomy

Origin

Lateral third of the clavicle, acromion and spine of the scapula.

Insertion

Deltoid tuberosity of the humerus.

Action

Abduction of the arm beyond the first 15°; anterior part flexes and medially rotates; posterior part extends and laterally rotates.

Innervation

Axillary nerve (posterior cord of the brachial plexus).

Nerve roots

C5–C6

Arterial supply

Posterior circumflex humeral artery; deltoid branch of the thoraco-acromial trunk; anterior circumflex humeral artery.

Relations

Covers the subacromial (subdeltoid) bursa, the rotator cuff tendons and the proximal humerus. The cephalic vein runs in the deltopectoral groove at its anterior border.

Function

  • Abducts the arm (15–90°)
  • Flexes and medially rotates (anterior part); extends and laterally rotates (posterior part)
  • Prevents inferior dislocation of the humeral head when carrying loads

Clinical significance

Axillary nerve injury

Caused by surgical neck fractures or anterior dislocation of the shoulder. It causes loss of abduction beyond 15°, flattening of the shoulder contour as deltoid wastes, and numbness over the lower deltoid ("regimental badge" area, via the superior lateral cutaneous nerve of the arm).

Intramuscular injection

Injections are given into the thickest part of deltoid, 2–3 finger-breadths (≈5 cm) below the acromion. Going lower risks the axillary and radial nerves.

Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.