The right deltoid muscle is a broad, triangular muscle that forms the rounded contour of the right shoulder. It lies beneath the skin, covers the shoulder joint and extends onto the upper arm. This article describes the whole deltoid; the same basic anatomy applies on the left side.
Where is the deltoid located?
The deltoid wraps around the front, outer side and back of the shoulder. It spans from the clavicle, or collarbone, and scapula, or shoulder blade, to the humerus, the bone of the upper arm. In the atlas, the right-sided muscle is grouped within the right pectoral girdle, the shoulder region that connects the upper limb to the trunk.
The deltoid is superficial, meaning close to the body surface. Its outer surface lies beneath skin, subcutaneous tissue—the tissue immediately under the skin—and fascia, a sheet of connective tissue surrounding muscles. Its deep surface faces the shoulder joint and neighbouring muscles.
The shoulder joint is a synovial joint, a joint whose cartilage-covered articular surfaces are separated by a cavity containing synovial fluid and enclosed by a capsule. Here, the rounded head of the humerus meets the glenoid cavity, the shallow socket of the scapula. The deltoid covers this region but does not form the joint itself.
What structures lie next to the deltoid?
The pectoralis major, the large muscle of the front of the chest, lies beside the anterior border of the deltoid. The interval between them is the deltopectoral groove. The cephalic vein, a superficial vein carrying blood from the upper limb towards the trunk, runs through this groove.
Deep to the deltoid are the rotator cuff muscles and their tendons. The rotator cuff is the group of muscles whose tendons surround the shoulder joint and help hold the humeral head against the socket. A tendon is the strong connective tissue through which a muscle attaches to another structure.
The subacromial bursa lies between the acromion and coracoacromial ligament above and the supraspinatus tendon below, and extends laterally beneath the deltoid. A bursa is a small fluid-containing sac that allows adjacent tissues to glide. The axillary nerve and posterior circumflex humeral vessels also pass deep to the muscle near the upper humerus.
What are the parts of the deltoid?
The deltoid has anterior, middle and posterior parts, named for their positions at the front, side and back of the shoulder. They are continuous portions of one muscle, rather than separate muscles. Their different attachments give their fibres different directions of pull.
| Part | Main attachment above | Main contribution to movement |
|---|---|---|
| Anterior | Lateral third of the clavicle | Moves the arm forwards and turns it inwards |
| Middle | Acromion of the scapula | Raises the arm sideways |
| Posterior | Spine of the scapula | Moves the arm backwards and turns it outwards |
The acromion is the bony projection of the scapula that forms the top of the shoulder. The spine of the scapula is the prominent ridge across its back surface. All these parts converge towards a shared attachment on the humerus.
Where does the deltoid originate and insert?
A muscle's origin is conventionally its more fixed attachment, while its insertion is usually the attachment that moves more during contraction. These terms describe the attachments; they do not mean that either bone must always remain still.
The deltoid originates from:
- The anterior border and upper surface of the lateral third of the clavicle.
- The lateral margin and upper surface of the acromion.
- The lower edge of the crest of the scapular spine.
It inserts into the deltoid tuberosity, a roughened area on the outer surface of the humeral shaft. These attachments allow the muscle to act between the bones of the shoulder girdle and upper arm, pulling the humerus in different directions according to which fibres contract.
What does the deltoid do?
The deltoid's main action is abduction, movement of the arm away from the side of the body. The middle part is especially important for this movement. The supraspinatus, a rotator cuff muscle above the spine of the scapula, assists in initiating and maintaining abduction.
The anterior part produces flexion, moving the arm forwards, and medial rotation, turning the humerus inwards. The posterior part produces extension, moving the arm backwards, and lateral rotation, turning the humerus outwards.
The deltoid and the rotator cuff muscles work together during shoulder movement. The deltoid supplies a strong lifting force, while the cuff helps control the position of the humeral head against the socket.
What supplies the deltoid with nerves and blood?
The axillary nerve supplies the deltoid. This nerve belongs to the peripheral nervous system, the nerves outside the brain and spinal cord, and carries motor fibres that activate the muscle. Its deltoid supply comes mainly from the C5 and C6 spinal nerve roots, the nerve connections emerging from the corresponding cervical spinal cord segments.
The axillary nerve passes through the quadrangular space with the posterior circumflex humeral artery and typically divides there into anterior and posterior branches. Its anterior branch continues around the surgical neck of the humerus, the narrowed region just below its upper end, deep to the deltoid.
Blood reaches the muscle principally through the posterior circumflex humeral artery and the deltoid branch of the thoracoacromial artery. The posterior circumflex humeral and thoracoacromial arteries arise from the axillary artery, the main artery of the armpit region.
How can you find the deltoid in the 3D model?
The atlas structure anchoring this article is named right deltoid muscle. This means the deltoid on the right side of the body, not one of its anterior, middle or posterior parts.
In the 3D model next to the article, rotate towards the outer side of the right shoulder, then select and isolate the muscle. Rotate between front and back views, and zoom in or out. You can then fade or hide the selected muscle, switch layers, or cut a section along the sagittal, coronal or transverse plane. You can also pull parts apart.