Muscle
Deltoid
Prime arm abductor
High-yield
Abducts arm 15–90°; axillary nerve injury (surgical neck fracture) causes deltoid paralysis and loss of the rounded shoulder.
Overview
The triangular muscle that caps the shoulder, giving it rounded contour. Its middle fibres are the main abductor beyond the first 15 degrees.
Reference facts
- Origin
Lateral third of clavicle, acromion, and spine of the scapula
- Insertion
Deltoid tuberosity of the humerus
- Action
Middle fibres abduct the arm; anterior fibres flex and medially rotate; posterior fibres extend and laterally rotate
- Innervation
Axillary nerve (C5, C6)
- Blood supply
Posterior circumflex humeral artery
Clinical correlation
Axillary nerve damage → deltoid wasting, weak abduction, and a patch of numbness over the lateral shoulder ('regimental badge').