The deltoid is the large, triangular muscle that gives the shoulder its rounded contour, wrapping from the clavicle and scapular spine around to a single attachment point on the lateral humerus. Its three distinct heads, anterior, middle, and posterior, are functionally independent enough to act in opposing directions, making the deltoid simultaneously a flexor, an abductor, and an extensor depending on which portion is active. It is the primary abductor of the arm once the rotator cuff muscles have initiated the first 15° of movement.
| Origin | Anterior border and superior surface of the lateral clavicle; Lateral margin and superior surface of the acromion; Inferior lip of the spine of the scapula |
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| Insertion | Deltoid tuberosity on the lateral surface of the humeral shaft |
| Nerve Supply | Axillary nerve (C5, C6) |
| Blood Supply | Anterior and posterior circumflex humeral arteries; Deltoid branch of thoracoacromial artery |
| Actions | Flexion and internal rotation of the arm; Abduction of the arm beyond 15°; Extension and external rotation of the arm; Stabilisation of the glenohumeral joint during loaded arm activities |
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The middle head is the workhorse for raising the arm out to the side, but all three heads contract together as powerful force generators during overhead pressing, and the anterior and posterior heads function as important stabilisers resisting shear forces during loaded activities.
Deltoid injuries are relatively uncommon in isolation because of its robust blood supply and broad attachments, but axillary nerve damage, from shoulder dislocation or proximal humeral fracture, can cause rapid deltoid atrophy and profound weakness in arm abduction. The anterior deltoid is frequently overloaded in athletes with pectoralis minor tightness who compensate with increased anterior shoulder activity during pressing. Posterior deltoid weakness is a common finding in overhead throwing athletes and contributes to posterior capsule tightness.
Each head is easily palpable with the arm in a position that isolates it, anterior head with resisted shoulder flexion, middle head with resisted abduction, and posterior head with resisted horizontal abduction or extension. The deltoid tuberosity insertion can be felt midway down the lateral humeral shaft.
Damage to the axillary nerve from shoulder dislocation or proximal humeral fracture produces loss of deltoid function and a patch of sensory loss over the lateral arm (the 'regimental badge' area), presenting as inability to abduct the arm against gravity.
Direct impact or eccentric overload of the deltoid produces localised pain, swelling, and weakness at the site of injury, most commonly the anterior head during heavy bench pressing or the posterior head during pulling exercises.
