Deltoid (*Delts)
Shoulder & Scapula · Upper Limb
The deltoid is the thick, triangular muscle that caps the shoulder and gives it its rounded contour. It is functionally three muscles in one: an anterior part from the lateral third of the clavicle, a middle part from the acromion, and a posterior part from the spine of the scapula, all of which converge on the deltoid tuberosity of the humerus. This tripartite arrangement lets the deltoid drive the arm in almost every direction. The anterior fibres flex and medially rotate it, the powerful middle fibres abduct it (taking over from supraspinatus past the first 15 degrees), and the posterior fibres extend and laterally rotate it. It is supplied by the axillary nerve (C5, C6), which winds around the surgical neck of the humerus. Because of this course, a fracture there or an anterior shoulder dislocation can injure the nerve, producing loss of abduction and a patch of numbness over the so-called regimental badge area.
Origin, Insertion, Action & Nerve
Origin
Anterior: lateral third of clavicle; Middle: acromion; Posterior: spine of scapula
Insertion
Deltoid tuberosity of humerus
Action
Anterior: flexion and medial rotation of arm; Middle: abduction of arm; Posterior: extension and lateral rotation of arm
Nerve
Axillary nerve (C5, C6)
Attachments explained
The Deltoid (*Delts) is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Anterior: lateral third of clavicle; Middle: acromion; Posterior: spine of scapula
Insertion
Deltoid tuberosity of humerus
Action & function
When the Deltoid (*Delts) contracts, it produces the following movements: Anterior: flexion and medial rotation of arm; Middle: abduction of arm; Posterior: extension and lateral rotation of arm.
As part of the Shoulder & Scapula group of the Upper Limb, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The Deltoid (*Delts) receives its nerve supply from the Axillary nerve (C5, C6). This nerve carries fibres from spinal segment(s) C5, C6.
Because a muscle can only contract when its nerve is intact, injury to the Axillary nerve or to its spinal roots (C5, C6) can weaken or paralyse the Deltoid (*Delts), impairing the movements it normally produces (flexion and medial rotation of arm). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Deltoid (*Delts) (the OIANS method)
Study the deltoid by its three parts rather than as one muscle. Anchor each part to its origin along the same bony line the trapezius uses (lateral clavicle, acromion, scapular spine), then predict its action from where those fibres sit relative to the shoulder's axis: fibres in front flex, fibres on the side abduct, and fibres behind extend. Reasoning from position is far more durable than rote listing.
Tie the deltoid to its clinical story to lock in the nerve. The axillary nerve hugs the surgical neck of the humerus, so a proximal humeral fracture or shoulder dislocation is the classic cause of deltoid paralysis and loss of the regimental badge sensation. If you can explain why abduction fails after a dislocation, you already know the origin, action, and nerve together.
Frequently asked questions
Where is the Deltoid (*Delts) located?
The Deltoid (*Delts) is a muscle of the Shoulder & Scapula group, located in the Upper Limb.
What is the origin of the Deltoid (*Delts)?
Anterior: lateral third of clavicle; Middle: acromion; Posterior: spine of scapula
What is the insertion of the Deltoid (*Delts)?
Deltoid tuberosity of humerus
What movements does the Deltoid (*Delts) produce?
Anterior: flexion and medial rotation of arm; Middle: abduction of arm; Posterior: extension and lateral rotation of arm
What nerve supplies the Deltoid (*Delts)?
Axillary nerve (C5, C6)
Is the Deltoid (*Delts) free to study in OIANS?
Yes. The Deltoid (*Delts) is free in OIANS: its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory, and because it belongs to the Upper Limb it is also covered by the free Quiz and Flashcard decks. The other regions, the "All Muscles" deck, and the Progress and Explore tools are unlocked with a one-time Lifetime purchase.
Related muscles
Study the Deltoid (*Delts)
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