Origin: Lateral one-third of clavicle, acromion, and spine of scapula
Insertion: Deltoid tuberosity of humerus
Action: Abducts, flexes, extends, medially and laterally rotates arm at glenohumeral (shoulder) joint
Innervation: Axillary nerve (C5-C6)
Arterial Supply: Acromial and deltoid branches of thoracoacromial artery, anterior and posterior circumflex humeral and subscapular arteries, deltoid branch of deep brachial artery
Key Features & Anatomical Relations
Structure: Triangular muscle with three heads – anterior (clavicular), middle (acromial), and posterior (scapular spinal)
Location:
Superficial to: Glenohumeral joint, proximal tendons of biceps brachii and triceps brachii muscles, axillary nerve
Deep to: Deltoid fascia, superior lateral brachial cutaneous nerve
Lateral to: Pectoralis major, trapezius, and infraspinatus muscles
Origin
Clavicular Part: Anterosuperior aspect of the lateral one-third of the clavicle
Acromial Part: Lateral aspect of the acromion of the scapula
Scapular Spinal Part: Inferior aspect of the spine of the scapula
Insertion
Description: Inferolaterally converging fibers forming a short, thick tendon
Location: Deltoid tuberosity of the humerus
Actions & Testing
Actions:
Acromial Part: Abducts arm at glenohumeral joint
Clavicular Part: Flexes and medially rotates arm at glenohumeral joint
Scapular Spinal Part: Extends and laterally rotates arm at glenohumeral joint
Testing: Abduct arm beyond 30° against resistance; muscle can be seen and palpated
Clinical Correlates
Intramuscular Injection Site: Commonly used due to muscle’s size and accessibility
