Deltoid
The deltoid is a large, triangular muscle that forms the rounded contour of the shoulder. It has three distinct parts (anterior, middle, posterior) that work together to produce a wide range of shoulder movements.
Origin, Insertion, Action, Innervation
OOrigin
Anterior fibers: Lateral third of clavicle. Middle fibers: Acromion of scapula. Posterior fibers: Spine of scapula.
IInsertion
Deltoid tuberosity of humerus
AAction
- • Abduction of the arm (middle fibers - main abductor after first 15°)
- • Flexion and medial rotation of the arm (anterior fibers)
- • Extension and lateral rotation of the arm (posterior fibers)
NInnervation
Axillary nerve (C5, C6)
Blood Supply
Posterior circumflex humeral artery and deltoid branch of thoracoacromial artery
Clinical Relevance
The axillary nerve can be damaged in shoulder dislocations or surgical neck fractures of the humerus. This causes weakness in abduction (especially between 15-90°) and sensory loss over the 'regimental badge' area. Deltoid is a common site for intramuscular injections.
Palpation
Easily palpated over the shoulder. Have the patient abduct the arm against resistance to feel the muscle contract. The three parts can be distinguished with careful palpation.
Study Tips
- ✓Three parts = three origins (clavicle, acromion, spine), one insertion
- ✓Axillary nerve wraps around surgical neck of humerus - vulnerable!
- ✓Supraspinatus initiates abduction (0-15°), deltoid continues (15-90°)
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Common questions about the deltoid
The deltoid has three parts: anterior (clavicular), middle (acromial), and posterior (spinal). Each part has different actions but they all insert together on the deltoid tuberosity of the humerus.
The deltoid is innervated by the axillary nerve, which carries fibers from the C5 and C6 nerve roots. This nerve wraps around the surgical neck of the humerus.
The deltoid (especially the middle fibers) is the primary abductor of the shoulder from 15° to 90°. Supraspinatus initiates abduction (0-15°), and after 90°, trapezius and serratus anterior rotate the scapula to continue the movement.
The regimental badge area is the skin over the lower part of the deltoid muscle. Sensory loss in this area indicates axillary nerve damage, which can occur with shoulder dislocations or humeral fractures.