Deltoid: origin, insertion, nerve supply and action
Also called: deltoid muscle, shoulder muscle
Quick answer
The deltoid is the thick triangular muscle that gives the shoulder its rounded contour. It arises from the lateral third of the clavicle, the acromion and the spine of the scapula and inserts on the deltoid tuberosity of the humerus. The axillary nerve supplies it, and it abducts the arm, with anterior and posterior fibres flexing and extending it.
Key facts
Origin
Anterior fibres: lateral third of the clavicle. Middle fibres: acromion. Posterior fibres: spine of the scapula.
Insertion
Deltoid tuberosity on the lateral shaft of the humerus.
Innervation
Axillary nerve (C5-C6).
Blood supply
Posterior circumflex humeral artery and the deltoid branch of the thoracoacromial artery.
Action
Middle fibres abduct the arm (from about 15 to 90 degrees, after supraspinatus starts the movement). Anterior fibres flex and medially rotate; posterior fibres extend and laterally rotate.
3D model: BodyParts3D (CC BY 4.0), adult male reference anatomy. Highlighted structure shown with nearby bones for context.
About the deltoid
The deltoid covers the shoulder joint on its anterior, lateral and posterior aspects. Its multipennate middle fibres give it great strength for abduction.
Its thickness makes the upper lateral part of the muscle a common site for intramuscular injections, keeping the needle well above the course of the axillary nerve.
Clinical relevance
The axillary nerve can be injured in fractures of the surgical neck of the humerus or anterior shoulder dislocation, causing deltoid weakness, a flattened shoulder and numbness over the lateral upper arm.
For injections, the safe zone is the thickest part of the muscle about two to three finger breadths below the acromion.
Frequently asked questions
What are the origin and insertion of the deltoid?
It arises from the lateral clavicle, the acromion and the spine of the scapula and inserts on the deltoid tuberosity of the humerus.
Which nerve supplies the deltoid?
The axillary nerve, which also supplies teres minor and the skin over the lower deltoid.
What is the main action of the deltoid?
Abduction of the arm at the shoulder, mainly by the middle fibres.
