Deltoid Muscle Anatomy

Location & Overview

The shape of the deltoid muscle is a defining feature of the human physique. It is the muscle which surrounds the shoulder joint forming a rounded contour. It is large and triangular in shape. The deltoid muscle is divided into three sets of distinct muscle fibres: the anterior head (clavicular fibers), intermediate/lateral head (acromial fibers) and the posterior head (spinal fibers) ^{[1] [2]}.

Origin & Insertion

The anterior clavicular fibres originate from the lateral third of the clavicle. The intermediate/lateral acromial fibres originate from the acromion. The posterior spinal fibres originate from the spine of the scapula. All the heads then converge into a single tendon insertion at the deltoid tuberosity of the humerus ^{[3] [4]}.

Actions

The actions of the deltoid muscle are shoulder flexion, extension and abduction. The different heads will assist more/less depending on the action. For example, during shoulder flexion, the anterior fibres will be utilised more. In abduction, the intermediate/lateral fibres will be utilised more. Finally, during extension of the shoulder, the posterior fibres will be utilised more. A contraction of all three heads simultaneously will result in abduction of the humerus ^{[5] [6] [7]}.

In addition to flexion, extension and abduction; the deltoid muscle can also assist the rotator cuff muscles with medial and lateral rotation of the humerus. The anterior fibres contribute to medial rotation and the posterior fibres contribute to lateral rotation ^[8].

Innervation

The deltoid muscle is innervated by the axillary nerve (C5, C6) ^{[9] [10]}.

Blood Supply

Blood is supplied to the deltoid by the thoracoacromial branch of the axillary artery. The posterior circumflex artery and the deltoid branches of the profunda brachii also provide minor contributions ^[11].