The deltoid is an intrinsic muscle of the shoulder region. It is shaped like an inverted triangle and can be divided into three parts (anterior, middle and posterior).
- Attachments: Originates from the lateral third of the clavicle, the acromion and the spine of the scapula. It attaches to the deltoid tuberosity on the lateral aspect of the humerus.
- Innervation: Axillary nerve.
- Anterior fibres – flexion and medial rotation at the shoulder
- Posterior fibres – extension and lateral rotation at the shoulder
- Middle fibres – abduction at the shoulder (takes over from the supraspinatus, which abducts the first 15 degrees).
- Blood supply: Anterior and posterior circumflex humeral artery.