On both sides, the deltoid muscles are important in attaching the shoulder girdle to the arm. In continuity, they originate from the:
- inferior surface of the lateral third of the clavicle
- spine of the scapula
They insert into the deltoid tuberosity. Acting as a unit, deltoid acts to abduct the arm at the glenohumeral joint. However, the anterior fibres assist in flexing and medially rotating the arm whereas the posterior fibres extend and laterally rotate.
Deltoid is innervated by the axillary nerve (C5, C6).