The middle (acromial) head of the deltoid is the largest and most powerful of the three deltoid heads, forming the rounded shoulder contour. Its multipennate architecture allows it to generate considerable force for arm abduction from 15 to 90 degrees, after which the supraspinatus contribution diminishes and the deltoid becomes the primary abductor. The middle head is the target of intramuscular injection in the deltoid.
| Origin | Lateral border and superior surface of the acromion |
|---|---|
| Insertion | Deltoid tuberosity of the humerus via the deltoid V |
| Nerve Supply | Axillary nerve (C5, C6) |
| Blood Supply | Posterior circumflex humeral artery; Deltoid branch of thoracoacromial artery |
| Actions | Primary abductor of the arm from 15 to 90 degrees; Works synergistically with supraspinatus in the initial phase of abduction |
|---|
Middle deltoid atrophy is the most visible sign of axillary nerve palsy, producing a flat shoulder profile and a positive drop arm test. Selective middle head wasting occurs in quadrilateral space syndrome and after proximal humeral fracture with axillary nerve traction. EMG of the middle deltoid is the most sensitive indicator of axillary nerve function. Intramuscular deltoid injections must target the middle head to avoid the axillary nerve inferiorly and the acromioclavicular joint superiorly.
Palpated with the arm held in 90 degrees abduction against resistance, where it forms a firm, rounded mass lateral to the acromion. The middle head is distinguished from the anterior and posterior heads by its direct lateral position.
Paralysis of the middle deltoid from axillary nerve injury in shoulder dislocation or proximal humeral fracture, producing a flat shoulder, absent abduction power, and a small patch of lateral arm numbness.
Partial tear of the acromial head from sudden forced adduction or eccentric overload, presenting as lateral shoulder pain and weakness with resisted abduction at 90 degrees.
