Deltoid Muscle: Origin, Insertion, Action & Innervation

1

Meet the Deltoid

The deltoid is the thick, triangular muscle that creates the rounded contour of the shoulder. Although it is one muscle, its fibers arise from three different regions and therefore have three different lines of pull. Those three parts are the clavicular (anterior), acromial (middle), and spinal (posterior) portions.

Isolated right deltoid muscle over the scapula and humerus

Isolated right deltoid. The acromial portion is highlighted in orange.

Learning objectives

Identify all three deltoid parts.

Describe each origin.

Locate the common insertion.

Match each part to its actions.

Name the innervating nerve.

Apply the anatomy during palpation.

Key idea: The deltoid’s actions change because each fiber group approaches the humerus from a different direction.

2

Origin, Insertion, and Action Explorer

Hover over or tap each card to reveal the information.

O

Origin

Where the muscle begins

Three origins

Anterior: anterior surface and superior border of the lateral one-third of the clavicle.
Middle: lateral margin and superior surface of the acromion.
Posterior: lower lip of the crest of the spine of the scapula.

I

Insertion

Where the fibers converge

Common insertion

All three parts converge into one tendon and attach to the deltoid tuberosity on the lateral humerus.

Because the insertion is shared, the direction of movement depends mainly on which fiber group is active.

A

Action

What contraction produces

Primary actions

Anterior: flexion, medial rotation, horizontal adduction.
Middle: abduction.
Posterior: extension, lateral rotation, horizontal abduction.

3

The Three Fiber Groups

Anterior / Clavicular

Origin: lateral one-third of the clavicle.

Shoulder flexion
Medial rotation
Horizontal adduction

Middle / Acromial

Origin: lateral margin and superior surface of the acromion.

Primary deltoid contribution to shoulder abduction
Strongest when the arm is raised laterally

Posterior / Spinal

Origin: lower lip of the spine of the scapula.

Shoulder extension
Lateral rotation
Horizontal abduction

Why the parts act differently

The anterior fibers lie in front of the shoulder joint and therefore pull the humerus forward and inward. The middle fibers pass almost directly over the lateral side of the joint, producing abduction. The posterior fibers lie behind the joint and pull the humerus backward and outward.

4

Insertion at the Deltoid Tuberosity

Deltoid insertion highlighted on the lateral humerus

The highlighted area on the lateral humerus represents the deltoid tuberosity.

One tendon, three lines of pull

The clavicular, acromial, and spinal fibers converge to form a common tendon. That tendon inserts onto the deltoid tuberosity, a roughened prominence along the lateral shaft of the humerus.

During contraction, the humerus moves toward the active origin. When the middle fibers contract, for example, the humerus is drawn upward and laterally into abduction.

Palpation landmark: The deltoid tuberosity is located approximately halfway down the lateral humerus and may be felt as a subtle roughened area beneath the distal deltoid.

5

Action in Motion

Movement demonstration supplied by the instructor. Replay the clip and observe how the humerus moves relative to the scapula.

Shoulder abduction

Abduction moves the arm laterally away from the body. The supraspinatus helps initiate the movement, and the middle deltoid becomes a major mover as elevation continues.

Normal overhead elevation also requires upward rotation of the scapula. The trapezius and serratus anterior are major contributors to that scapular motion, while the rotator cuff helps center the humeral head in the glenoid cavity.

Clinical observation: If the axillary nerve or deltoid is impaired, the client may struggle to raise the arm laterally against resistance.

6

Innervation: The Axillary Nerve

The deltoid is supplied by the axillary nerve, a branch of the posterior cord of the brachial plexus. Its primary spinal nerve roots are C5 and C6. The nerve travels around the surgical neck of the humerus and gives muscular branches to the deltoid.

Safety connection: The axillary nerve is vulnerable near the surgical neck of the humerus. Injury may produce deltoid weakness, reduced shoulder abduction, and flattening of the normal shoulder contour.

7

Quick Reference

Part	Origin	Insertion	Main actions
Anterior	Lateral one-third of clavicle	Deltoid tuberosity of humerus	Flexion, medial rotation, horizontal adduction
Middle	Acromion		Abduction
Posterior	Spine of scapula		Extension, lateral rotation, horizontal abduction

Palpate anterior fibersAsk for gentle shoulder flexion against resistance.

Palpate middle fibersAsk for gentle shoulder abduction against resistance.

Palpate posterior fibersAsk for gentle shoulder extension or horizontal abduction.

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Deltoid Muscle