Acromial part of deltoid muscle

At a glance

English

Acromial part of deltoid muscle

Latin (TA2)

Pars acromialis musculi deltoidei

System

Muscle

Region

Upper limb

Side

Paired, left and right

About the acromial part of deltoid muscle

The deltoid caps the shoulder and gives it its rounded shape. Its fibres fall into three parts named after where they arise: a clavicular part in front, a spinal part behind, and between them the acromial part, also called the middle or lateral deltoid. The acromial part lies directly over the outer side of the shoulder joint, with the subacromial (subdeltoid) bursa between it and the rotator cuff.

Its fibres begin on the lateral edge and upper surface of the acromion, the flat outer end of the scapular spine. They run straight down over the head of the humerus and converge on the deltoid tuberosity, the roughened area halfway down the outer side of the humeral shaft. Unlike the front and back parts, whose fibres run in parallel, the middle part is multipennate: short fibres are packed obliquely between several tendinous sheets. This design gives a large force over a short range, matching its role as the main abductor.

The acromial part raises the arm sideways at the shoulder. Supraspinatus helps it to start the movement, and the deltoid takes over as the main abductor from about fifteen degrees up to the horizontal; above that, rotation of the scapula by trapezius and serratus anterior is needed. As it lifts the arm, the deltoid also pulls the humeral head upward, and the rotator cuff muscles must hold the head down in the socket to stop it pressing on the acromion. The middle fibres also resist downward dislocation when heavy loads are carried. The whole deltoid is supplied by the axillary nerve, from the fifth and sixth cervical segments, and the middle part receives blood mainly from the posterior circumflex humeral artery and the acromial branch of the thoracoacromial artery.

Injury to the axillary nerve, for example in a dislocation of the shoulder or a fracture of the surgical neck of the humerus, weakens abduction and flattens the rounded shoulder contour, and causes numbness over a patch of skin on the outer upper arm. The deltoid is also a common site for injections given into muscle.

Attachments, action and supply

Origin

Lateral margin and upper surface of the acromion.

Insertion

Deltoid tuberosity on the lateral surface of the humeral shaft.

Action

Abducts the arm at the shoulder, chiefly from about fifteen degrees to the horizontal; steadies the humeral head when loads are carried.

Nerve supply

Axillary nerve (C5, C6).

Blood supply

Posterior circumflex humeral artery and the acromial branch of the thoracoacromial artery.