Thick, triangular muscle. Covers upper half of humerus anteriorly, laterally, and posteriorly. Forms rounded contour of shoulder.

Origin, Insertion and Architecture of Deltoid

Origin

Insertion

All three parts → deltoid tuberosity on lateral shaft of humerus (via three septa of insertion)

Architecture — Multipennate Arrangement

• Acromial (middle) part = classic example of multipennate muscle
• Four septa of origin (acromion) → fibres converge onto three septa of insertion (deltoid tuberosity)
• Packs large number of short fibres into small volume → strength ∝ number of fibres, not length
• Multipennate acromial part = strongest part of deltoid

Nerve Supply and Actions of Deltoid

Nerve Supply

Axillary nerve (C5, C6)

Actions — “Three Muscles in One”

Abduction — The 0°–15° Problem

• Acromial fibres cannot initiate abduction from 0°–15° — vertical pull aligns with long axis of arm → no abductor moment
• First 15°: supraspinatus initiates
• 15°–90°: deltoid continues
• Beyond 90°: scapular rotation (trapezius + serratus anterior)

Clinical Testing

Resisted abduction + palpate contracting muscle

Structures Deep to (Under Cover of) Deltoid

Bones: Upper end of humerus; coracoid process

Joint and Ligaments: Shoulder (glenohumeral) joint; coracoacromial ligament

Bursae:

• Subacromial (subdeltoid) bursa — largest bursa in body; below coracoacromial arch, above supraspinatus tendon and greater tubercle; protects supraspinatus from friction
• Subscapular bursa; infraspinatus bursa

Muscles — Insertions:

• Supraspinatus, infraspinatus, teres minor → greater tubercle
• Subscapularis → lesser tubercle
• Pectoralis major, teres major, latissimus dorsi → intertubercular sulcus
• Pectoralis minor → coracoid process

Muscles — Origins:

• Coracobrachialis + short head of biceps → coracoid process
• Long head of biceps → supraglenoid tubercle
• Long head + lateral head of triceps → infraglenoid tubercle and posterior humerus

Vessels: Anterior and posterior circumflex humeral artery

Nerve: Axillary nerve + posterior circumflex humeral vessels on deep aspect of deltoid

Spaces: Quadrangular space; triangular (subscapular) intermuscular space

Applied Anatomy of Deltoid

Axillary Nerve Injury

Causes: Dislocation of shoulder joint; fracture of surgical neck of humerus

Effects:

• Loss of rounded contour of shoulder; greater tubercle becomes prominent
• Deltoid paralysed → loss of abduction 0°–90°
• Sensory loss over lower half of deltoid = “regimental badge” area (upper lateral cutaneous nerve of arm, branch of axillary nerve)

Intramuscular Injection into Deltoid

• Given into upper and outer quadrant of deltoid (lower half, away from axillary nerve course)
• Avoids injury to axillary nerve winding around surgical neck of humerus

Dawbarn’s Sign (Subacromial / Subdeltoid Bursitis)

• Pressure below acromion with arm by side → pain
• Arm abducted → pressure on same point → no pain (inflamed bursa disappears under acromion)
• Positive Dawbarn’s sign = subacromial/subdeltoid bursitis (usually secondary to supraspinatus tendinitis)

Summary — Clinical Correlations

Origin, Insertion and Architecture of Deltoid

The deltoid is a thick, triangular muscle that covers the upper half of the humerus on its anterior, lateral, and posterior aspects, forming the rounded contour of the shoulder.

Origin

The deltoid arises from three contiguous regions, corresponding to its three functional parts:

Insertion

All three parts converge to insert into the deltoid tuberosity on the lateral aspect of the shaft of the humerus, via three septa of insertion.

Architecture — Multipennate Arrangement

• The acromial (middle) part of deltoid is a classic example of a multipennate muscle.
• Numerous muscle fibres arise from four septa of origin attached to the acromion above, and converge onto three septa of insertion attached to the deltoid tuberosity below.
• This arrangement packs a large number of short muscle fibres into a relatively small muscle volume.
• Since the strength of contraction of a muscle is proportional to the number of muscle fibres (not their length), the multipennate acromial part is much stronger than a muscle of similar volume with a simpler architecture — making it the strongest part of the deltoid.

Functional Note (Architecture-Based)

This multipennate design underlies why the acromial fibres act as the powerful abductor component of the deltoid (detailed under actions), while the anterior and posterior parts retain a simpler, less pennate arrangement suited to their roles as flexor/medial rotator and extensor/lateral rotator respectively.

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Nerve Supply and Actions of Deltoid

Nerve Supply

The deltoid is supplied by the axillary nerve (C5, C6).

Actions — “Three Muscles in One”

The deltoid functions as three muscles in one, with each of its three parts producing a different action at the shoulder joint:

Abduction — The 0°–15° Problem

• The multipennate acromial fibres are powerful abductors of the arm from 15° to 90°.
• These fibres cannot initiate abduction from 0° to 15° when the arm is by the side of the body, because at this position the vertical pull of the acromial fibres corresponds to the long axis of the arm — there is no abductor moment.
• The first 15° of abduction is initiated by the supraspinatus, after which the deltoid takes over and continues abduction up to 90°.
• Beyond 90°, further elevation occurs through scapular rotation (scapulohumeral rhythm), involving trapezius and serratus anterior.

Clinical Testing

• The deltoid can be tested by asking the patient to abduct the arm against resistance, while the examiner palpates the contracting muscle with the other hand.
• The muscle can be easily seen and felt to contract during resisted abduction.

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Structures Deep to (Under Cover of) Deltoid

The deltoid covers the shoulder joint and a large number of important structures. These can be organised by category.

Bones

• Upper end of the humerus
• Coracoid process of the scapula

Joint and Ligaments

• Shoulder (glenohumeral) joint
• Coracoacromial ligament

Bursae

All bursae around the shoulder joint, including:

• Subacromial (subdeltoid) bursa — the largest bursa in the body, situated below the coracoacromial arch and the deltoid, above the supraspinatus tendon and greater tubercle of the humerus
• Subscapular bursa
• Infraspinatus bursa

Functional Role of the Subacromial Bursa

• Protects the supraspinatus tendon from friction against the acromion.
• During overhead abduction, facilitates smooth passage of the greater tubercle of the humerus under the acromion.

Muscles

Insertions (muscles inserting on the humerus, covered by deltoid)

• Pectoralis minor — on the coracoid process
• Supraspinatus, infraspinatus, and teres minor — on the greater tubercle of the humerus
• Subscapularis — on the lesser tubercle of the humerus
• Pectoralis major, teres major, and latissimus dorsi — on the intertubercular sulcus (bicipital groove) of the humerus

Origins (muscles arising from structures under deltoid)

• Coracobrachialis and short head of biceps brachii — from the coracoid process
• Long head of biceps brachii — from the supraglenoid tubercle
• Long head of triceps brachii — from the infraglenoid tubercle
• Lateral head of triceps brachii — from the upper part of the posterior surface of the humerus

Vessels

• Anterior circumflex humeral artery
• Posterior circumflex humeral artery (and vein)

Nerve

• Axillary nerve, accompanied by the posterior circumflex humeral vessels, lies on the deep aspect of the deltoid as it winds around the surgical neck of the humerus.

Spaces

• Quadrangular space
• Triangular (subscapular) intermuscular space

Musculotendinous Cuff (Rotator Cuff) — Brief Note

The flattened tendons of subscapularis, supraspinatus, infraspinatus, and teres minor blend with the capsule of the shoulder joint, forming the rotator cuff, which strengthens the capsule on all sides except inferiorly. This cuff lies deep to the deltoid and crosses the shoulder joint before the respective insertions listed above.

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Applied Anatomy of Deltoid

Axillary Nerve Injury

The axillary nerve, which lies on the deep surface of the deltoid as it winds around the surgical neck of the humerus, may be damaged by:

• Dislocation of the shoulder joint
• Fracture of the surgical neck of the humerus

Effects of Axillary Nerve Injury

• The rounded contour of the shoulder is lost, and the greater tubercle of the humerus becomes prominent, since the deltoid (which normally caps the shoulder) is paralysed and wasted.
• Deltoid is paralysed, resulting in loss of the power of abduction of the arm from 0° to 90° at the shoulder joint.
• Sensory loss occurs over the lower half of the deltoid, in a characteristic patch of skin known as the “regimental badge” area — corresponding to the cutaneous distribution of the upper lateral cutaneous nerve of the arm (a branch of the axillary nerve).

Intramuscular Injection into Deltoid

• Intramuscular injections are commonly given into the deltoid.
• To avoid injury to the axillary nerve (which winds around the surgical neck of the humerus deep to the muscle), injections should be given into the upper and outer quadrant of the deltoid region (lower half of deltoid, away from the nerve’s course).

Dawbarn’s Sign (Subacromial / Subdeltoid Bursitis)

• In subacromial bursitis, pressure applied over the deltoid just below the acromion, with the arm hanging by the side, causes pain.
• When the arm is then abducted, pressure over the same point causes no pain, because the inflamed bursa disappears under cover of the acromion during abduction.
• This change in tenderness with arm position is known as Dawbarn’s sign, and is suggestive of subacromial/subdeltoid bursitis — usually secondary to supraspinatus tendon inflammation.

Summary — Clinical Correlations

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