[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anatomy](https://durgical.com/en/anatomy/) › [Upper limb](https://durgical.com/en/anatomy/region/upper-limb/) › Middle deltoid [Muscular system](https://durgical.com/en/anatomy/system/muscular-system/) · [Upper limb](https://durgical.com/en/anatomy/region/upper-limb/) # Middle deltoid The middle deltoid is the outer part of the shoulder's rounded muscle; it is the main muscle that lifts the arm out to the side. Official name: Deltoid, middle (acromial) part [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=upper_limb&select=deltoid_middle&side=r) Opens in the Durgical app, no sign-up needed. ## What it is The middle part of the rounded shoulder muscle, over the side of the shoulder. ## What it does Lifts the arm out to the side. ## Key points - The acromial part is multipennate, unlike the front and rear parts: many short fibres give the strongest pull for abduction - The axillary nerve winds round the surgical neck on its deep surface, roughly 5 cm below the acromion (the distance varies); deltoid splits are kept above it - Injections go into its upper, thickest part a few finger-widths below the acromion; too low risks the axillary nerve, too far back and low the [radial nerve](https://durgical.com/en/anatomy/radial-nerve/) ## Key facts **Origin** Acromion **Insertion** Deltoid tuberosity of the [humerus](https://durgical.com/en/anatomy/humerus/) **Action** Abducts the arm (after the first ~15°) **Nerve** Axillary nerve (C5–C6) **Blood supply** Posterior circumflex humeral artery, [thoracoacromial artery](https://durgical.com/en/anatomy/thoracoacromial-artery/) (deltoid branch) ## Relations **Above** Lateral edge of the acromion **Deep** The subacromial (subdeltoid) bursa over the [supraspinatus tendon](https://durgical.com/en/anatomy/supraspinatus-tendon/) and greater tubercle; lower, the [axillary nerve](https://durgical.com/en/anatomy/axillary-nerve/) and posterior circumflex humeral vessels **In front** [Front deltoid](https://durgical.com/en/anatomy/deltoid-anterior-part/) **Behind** [Rear deltoid](https://durgical.com/en/anatomy/deltoid-posterior-part/) **Superficial** [Skin](https://durgical.com/en/anatomy/skin/) of the outer shoulder, supplied by the upper lateral cutaneous nerve of the arm (the 'regimental badge' patch) ## How to find it Abducting the arm to about 90° against resistance hardens the rounded mass below the lateral edge of the acromion, the bony shelf felt at the top of the shoulder. ## Examination - Regimental badge sensation over the lower outer deltoid, checked before and after reducing a dislocated shoulder: numbness suggests axillary nerve injury, but normal feeling does not rule out motor damage - Resisted abduction at 90°: weakness with a normal start of abduction (first ~15°) points to the axillary nerve or the C5 root rather than the [supraspinatus](https://durgical.com/en/anatomy/supraspinatus/) ## On imaging On coronal MRI it is the thick muscle over the greater tuberosity, with the [subacromial-subdeltoid bursa](https://durgical.com/en/anatomy/subacromial-subdeltoid-bursa/) beneath it. On ultrasound, when the supraspinatus tendon is torn through and absent, the deltoid lies directly on the humeral head: a classic sign of a full-thickness tear. Axillary nerve injury shows as deltoid atrophy and fatty change on MRI. ## Common problems - Weakness or wasting when the axillary nerve is injured (shoulder dislocation, humerus fracture). - In a reverse shoulder replacement it becomes the main muscle that lifts the arm. ## How it is diagnosed Exam of shoulder lifting strength and of the skin feeling over the outer shoulder; EMG if a nerve injury is suspected. ## Treatment - Physical therapy to strengthen it. - Its strength is what makes a reverse shoulder replacement work. ## Clinical notes - Lateral deltoid-splitting approaches to the shoulder and proximal humerus stop or protect the axillary nerve (texts quote about 5 cm below the acromion, with wide variation) - Supraspinatus and middle deltoid work as a force couple: without the cuff, the deltoid pulls the humeral head up against the acromion instead of rotating it - Deltoid contracture after repeated childhood injections is a recognised cause of a fixed abducted shoulder with a dimple over the muscle ## Development It forms in place from the mesoderm of the arm bud and belongs to the dorsal muscle mass, which is why its nerve, the axillary nerve, comes from the [posterior cord of the brachial plexus](https://durgical.com/en/anatomy/posterior-cord-of-the-brachial-plexus/). ## Related conditions - [Lambert-Eaton myasthenic syndrome](https://durgical.com/en/conditions/lambert-eaton-myasthenic-syndrome/) - [Periodic paralyses (hypokalaemic, hyperkalaemic, thyrotoxic)](https://durgical.com/en/conditions/periodic-paralyses/) - [Inflammatory myopathies (dermatomyositis, polymyositis)](https://durgical.com/en/conditions/inflammatory-myopathies/) - [Axillary nerve injury](https://durgical.com/en/conditions/axillary-nerve-injury/) ## Test yourself Think of the answer, then tap the question to see it. ## Why is the middle deltoid the strongest abductor of the three deltoid parts? It is multipennate: many short fibres give the strongest pull for abduction. ## How far below the acromion does the axillary nerve run deep to the middle deltoid? Roughly 5 cm, winding round the surgical neck; the distance varies, so deltoid splits are kept above it. ## What does the regimental badge test check after a shoulder dislocation, and what is its limitation? Sensation over the lower outer deltoid (axillary nerve); numbness suggests injury, but normal feeling does not rule out motor damage. ## Why does the humeral head ride up against the acromion when the rotator cuff fails? The supraspinatus and middle deltoid form a force couple; without the cuff the deltoid pulls the head upward instead of rotating it. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anatomy/deltoid-middle-part/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** pars acromialis musculi deltoidei TA2 2454 **Official Spanish (DeCS)** Porción acromial del músculo deltoides **English (TA2)** Deltoid, middle (acromial) part ## Related - [Axillary nerve](https://durgical.com/en/anatomy/axillary-nerve/) - [Circumflex humeral arteries](https://durgical.com/en/anatomy/anterior-and-posterior-circumflex-humeral-arteries/) - [Humerus](https://durgical.com/en/anatomy/humerus/) - [Scapula](https://durgical.com/en/anatomy/scapula/) Sources: Terminologia Anatomica (TA2); [Anatomy, Shoulder and Upper Limb, Deltoid Muscle](https://www.ncbi.nlm.nih.gov/books/NBK537056/); Anatomy of the Human Body (20th ed.): Development of the Muscles (Lea & Febiger (Gray's Anatomy, 1918), 1918) · adapted, Public domain. ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/anatomy/deltoid-middle-part/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/deltoid-middle-part/