Deltoid

Origin, Insertion, Action, Innervation

Origin:
Anterior fibers: Lateral third of the clavicle
Middle fibers: Acromion process
Posterior fibers: Spine of the scapula
Insertion: Deltoid tuberosity on the lateral surface of the humeral shaft (approximately midshaft)
Action:
Anterior fibers: Flexion of the shoulder, horizontal adduction, medial rotation (minor)
Middle fibers: Abduction of the shoulder (primary mover from 15°–90°)
Posterior fibers: Extension of the shoulder, horizontal abduction, lateral rotation (minor)
All fibers contribute to shoulder abduction once the movement is initiated by supraspinatus
Innervation: Axillary nerve (C5–C6)

Palpation Guide

Client position: Seated or supine.
Landmark sequence:

The deltoid is immediately visible and palpable covering the lateral shoulder. Place your hand on the rounded contour of the shoulder — you are on the deltoid.
Anterior fibers: Trace from the lateral clavicle inferiorly and laterally to the deltoid tuberosity. These fibers lie anterior to the acromion.
Middle fibers: Palpate directly lateral to the acromion, running from the acromion to the deltoid tuberosity. This is the thickest part of the muscle.
Posterior fibers: Trace from the scapular spine inferiorly and laterally to the deltoid tuberosity. These fibers lie posterior to the acromion.
The deltoid tuberosity (insertion) is palpable on the lateral humerus approximately midway down the shaft — a slightly roughened area.

Tissue feel: Thick and fleshy — one of the most substantial muscles of the upper body. The middle fibers form the bulk of the shoulder contour. In a well-developed individual, the three portions are visually distinct. In atrophy (axillary nerve lesion), the shoulder appears flattened and squared-off rather than rounded.
Confirmation test: Ask the client to abduct the arm to 90° against your resistance. All three portions contract, with the middle fibers most prominent. To isolate anterior fibers, resist forward flexion. To isolate posterior fibers, resist extension or horizontal abduction.
Common errors:
Confusing the anterior deltoid with pectoralis major — the clavicular head of pec major lies medial to the anterior deltoid. The deltopectoral groove (a visible and palpable groove between the two muscles) marks the border.
Missing the posterior deltoid — students focus on the anterior and middle portions and neglect the posterior fibers, which harbor TrPs in clients with posterior shoulder pain.
Failing to recognize deltoid atrophy — a flattened, squared-off shoulder contour suggests axillary nerve compromise.

Trigger Point Referral

Common TrP locations: Anterior fibers have TrPs near the clavicular attachment. Middle fibers have TrPs in the mid-belly. Posterior fibers have TrPs along the scapular spine attachment.
Referral pattern: All three portions refer locally — the anterior fibers refer to the anterior and lateral deltoid region, the middle fibers refer laterally, and the posterior fibers refer to the posterior deltoid. The referral is relatively localized compared to muscles like the infraspinatus or upper trapezius.
Clinical significance: Deltoid TrPs produce shoulder pain that is easily confused with subacromial impingement. The key differentiator: deltoid TrP pain is reproduced by direct palpation of the TrP, whereas impingement pain is provoked by specific impingement tests (Neer, Hawkins-Kennedy) and arc of motion.

Trigger point referral diagram — coming soon

Image coming soon. For visual reference, see [Deltoid at TriggerPoints.net](http://www.triggerpoints.net/muscle/deltoid).

Clinical Notes

Innervation significance:

The axillary nerve (C5–C6) wraps around the surgical neck of the humerus through the quadrilateral space. This makes it vulnerable to humeral fractures and shoulder dislocations. Deltoid weakness (loss of shoulder abduction and contour) is the hallmark sign of axillary nerve injury.

Common conditions:

Deltoid strain from overuse in overhead sports and repetitive arm elevation — pain at the deltoid tuberosity (insertion) or the acromial origin.
Deltoid TrPs develop secondary to conditions/subacromial-impingement and rotator cuff dysfunction — the deltoid compensates for rotator cuff weakness by increasing its firing, leading to overuse TrPs.
Axillary nerve injury from shoulder dislocation or humeral neck fracture produces deltoid paralysis — loss of the rounded shoulder contour and inability to abduct the arm.
Deltoid inhibition from pain or nerve injury contributes to altered shoulder mechanics in conditions/rotator-cuff-tendinopathy.

What you'll typically find:

In clients with shoulder pain, the deltoid is often tender from overuse but is rarely the primary pathology. It is typically compensating for underlying rotator cuff dysfunction. Always assess the rotator cuff (supraspinatus, infraspinatus, subscapularis) before focusing on the deltoid.
The posterior deltoid frequently harbors TrPs in clients who work with their arms in front of them (desk workers, drivers) — the posterior fibers are eccentrically loaded during sustained forward arm positions.
Atrophy of the deltoid (visible flattening of the shoulder contour) is an important clinical sign — it suggests axillary nerve compromise and requires referral.

Treatment effects:

Responds well to longitudinal stripping along each fiber group. Treat the anterior, middle, and posterior portions as three separate muscles, stripping from origin to insertion along their respective fiber directions.
TrPs respond to sustained compression — the deltoid is thick enough to tolerate firm pressure. Hold for 30–60 seconds per TrP.
Post-treatment, shoulder ROM typically improves, particularly abduction and horizontal movements.

Cautions:

The axillary nerve wraps around the surgical neck of the humerus, approximately 5–7 cm below the acromion. Avoid aggressive deep pressure on the lateral humerus at this level, particularly in clients with thin musculature.
The posterior circumflex humeral artery accompanies the axillary nerve through the quadrilateral space.
In clients post-shoulder dislocation, palpate carefully — the tissue may be bruised and the nerve may be compromised.

Postural significance:

The deltoid is not a primary postural muscle but is affected by postural patterns. In upper crossed syndrome, the anterior deltoid shortens along with pectoralis major (both are horizontal adductors and flexors), contributing to internally rotated, protracted shoulders.

Clinical pearl:

The deltoid and supraspinatus work together for the first 90° of abduction — supraspinatus initiates and deltoid takes over. If a client has pain only during the initial 0°–15° of abduction, suspect supraspinatus. If pain is throughout the 15°–90° range, the deltoid is more likely involved. If both are painful, both muscles need treatment.

Assessment

Manual muscle testing:

Shoulder abduction (middle fibers): Client seated with the arm abducted to 90°, elbow flexed. Apply downward pressure on the arm. Grade bilaterally.
Shoulder flexion (anterior fibers): Client seated. Resist forward flexion at approximately 90°.
Shoulder extension (posterior fibers): Client prone. Resist extension against gravity.

Stretch test:

Horizontal adduction (posterior fibers): Client seated. Horizontally adduct the arm across the body. Restriction suggests tight posterior deltoid.
Extension (anterior fibers): Client seated. Extend the arm behind the body. Restriction suggests tight anterior deltoid.

Related special orthopedic tests:

Neer impingement test — to differentiate deltoid pain from subacromial impingement
Hawkins-Kennedy test — subacromial impingement
Empty can test — supraspinatus isolation

Muscle Groups

Shoulder abductors (functional):

Deltoid (this article) — middle fibers; primary mover 15°–90°
anatomy/muscles/supraspinatus — initiates abduction 0°–15°

Shoulder flexors (functional):

Deltoid (this article) — anterior fibers
anatomy/muscles/pectoralis-major — clavicular head
Coracobrachialis
Biceps brachii (long head)

Shoulder extensors (functional):

Deltoid (this article) — posterior fibers
Latissimus dorsi
anatomy/muscles/teres-major

Rotator cuff + deltoid — shoulder abduction force couple (clinical):

Axillary nerve group (innervation):

Deltoid (this article)
anatomy/muscles/teres-minor

Related Muscles

Synergists for shoulder abduction:

anatomy/muscles/supraspinatus — initiates abduction; the two work as a force couple
anatomy/muscles/upper-trapezius — upwardly rotates the scapula to allow abduction beyond 90°

Antagonists:

anatomy/muscles/pectoralis-major — adducts the shoulder
Latissimus dorsi — adducts, extends, and internally rotates
anatomy/muscles/teres-major — adducts and internally rotates

Same innervation (axillary nerve):

anatomy/muscles/teres-minor — shares C5–C6 axillary nerve innervation; lateral rotator of the shoulder

Key Takeaways

Three-part muscle (anterior, middle, posterior) — each portion has a different action, and each must be assessed separately.
Axillary nerve vulnerability at the surgical neck of the humerus means deltoid weakness (flattened shoulder contour) is a red-flag sign requiring referral.
Deltoid TrP pain mimics impingement — differentiate by palpation (TrP reproduction) vs. impingement tests (arc of motion reproduction).