Intrinsic Muscles of the Shoulder

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Introduction

The intrinsic muscles of the shoulder form the scapulohumeral group, mainly originating from the scapula and inserting onto the humerus.¹

The intrinsic muscles can be further divided into the deltoid muscle, teres major and the rotator cuff muscles; supraspinatus, infraspinatus, subscapularis and teres minor.²

This article will primarily highlight the origin, insertion, innervation and function of the intrinsic muscles of the shoulder.

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Deltoid

The deltoid forms the rounded contour of the shoulder and is comprised anterior, intermediate and posterior fibres.¹

Origin and insertion

The anterior (or clavicular) fibres originate from the lateral third of the clavicle. The intermediate (or acromial) fibres originate from the acromion process of the scapula. The posterior (or spinal) fibres originate from the spine of the scapula.

All fibres converge toward their insertion on the deltoid tuberosity on the middle of the shaft of the humerus.

Innervation

The deltoid is innervated by the axillary nerve (C5-6).

Function

The functions of the deltoid muscle can be categorised based on the groups of fibres involved:

Anterior fibres: shoulder flexion and internal rotation.
Intermediate fibres: abduction of the shoulder (these fibres take over from supraspinatus which initiates shoulder abduction).
Posterior fibres: shoulder extension and external rotation.

Teres major

Teres major is a thick muscle which is positioned superiorly to latissimus dorsi.⁴

Origin and insertion

Teres major originates from the posterior aspect of the inferior angle of the scapula and inserts onto the medial lip of the bicipital groove (intertubercular sulcus) of the humerus.⁴

Innervation

The majority of teres major is innervated by the lower subscapular nerve and a small portion is innervated by the thoracodorsal nerve (C5-8).⁴

Function

Teres major acts to adduct the humerus as well as assisting in extension and internal rotation of the shoulder.⁴

Rotator cuff muscles

All four rotator cuff muscles originate from areas of the scapula and insert onto the humeral head.²

The glenohumeral (GH) joint is highly mobile, allowing a large range of movement, but this comes at the cost of stability. The resting tension of the rotator cuff muscles provides the primary stabilizing force on the GH joint.^1,2

Supraspinatus

Supraspinatus arises from the scapula and runs beneath the cover of the acromion.⁵

Origin and insertion

Supraspinatus originates from the supraspinous fossa and the muscle’s tendon inserts onto the superior facet of the greater tubercle of the humerus.⁵

Innervation

Supraspinatus is innervated by the suprascapular nerve (C5).⁵

Function

Supraspinatus acts to initiate abduction of the arm (especially between the first 15 degrees of movement) and then acts to assist the deltoid muscle (15-90 degrees of movement).⁵

Clinical relevance: impingement syndrome

Impingement syndrome is caused by rotator cuff tendonitis as the tendons pass beneath the acromion. The supraspinatus muscle’s tendon is most commonly affected.

Impingement syndrome typically presents with pain, weakness and restricted shoulder movement. Patients with impingement syndrome often complain of pain when their arms are raised (this is particularly common in mechanics and manual labourers who work with their arms overhead). When the arm is raised, the subacromial space narrows, which can result in impingement of the supraspinatus muscle tendon leading to an inflammatory response.

Clinical examination

Typical findings on clinical examination in supraspinatus impingement syndrome include:

Pain experienced between 60-120° of shoulder abduction (known as a ‘painful arc’).
Weakness and pain experienced when the supraspinatus muscle is isolated using the ‘Empty can/Jobe’s test

For more information, see the Geeky Medics shoulder examination guide.

Infraspinatus

Infraspinatus is a thick triangular muscle. The spine of the scapula acts to separate the origin of the supraspinatus muscle from the infraspinatus muscle.⁶

Origin and insertion

The infraspinatus muscle originates from the infraspinous fossa of the scapula, passing over the lateral border of the spine of the scapula before inserting onto the middle facet of the greater tuberosity on the humerus.⁶

Innervation

Infraspinatus has the same innervation as supraspinatus – the suprascapular nerve (C5).⁶

Function

Infraspinatus acts to externally rotate the arm.⁶

Subscapularis

Subscapularis is another large triangular-shaped muscle.⁷

Origin and insertion

It originates from the subscapular fossa (ventral surface) and inserts onto the lesser tuberosity of the humerus (this is the only rotator cuff muscle which doesn’t attach to the greater tuberosity).⁷

Innervation

Subscapularis is innervated by the upper and lower subscapular nerves (C5-6).⁷

Function

Subscapularis acts to internally rotate and adduct the arm.⁷

Teres minor

Teres minor is a narrow elongated muscle which is superior to teres major.⁸

Origin and insertion

Teres minor originates from the lateral border of the scapula, before inserting onto the inferior facet of the greater tubercle on the humerus.⁸

Innervation

Teres minor is innervated by the axillary nerve (C5-6).⁸

Function

Teres minor acts to externally rotate the arm.⁸

Mnemonic: SIT

SIT is a mnemonic to remember the superior to inferior attachments of the rotator cuff muscles onto the greater tubercle of the humerus.⁵

Supraspinatus
Infraspinatus
Teres minor

Summary

Muscle	Origin	Insertion	Innervation	Action
Deltoid	Lateral ⅓ clavicle + acromion & the spine of the scapula	Deltoid tuberosity	Axillary nerve	Anterior fibres: Flexion & internal rotation Intermediate fibres: major abductor, takes over from supraspinatus Posterior fibres: extension & external rotation
Teres major	Inferior angle of the scapula	Medial lip of bicipital groove	Lower subscapular nerve	Adducts & extends at the shoulder Internally rotates the arm
Rotator cuff muscles
Supraspinatus	Supraspinous fossa	Greater tubercle	Suprascapular nerve	Initiates abduction (0-15 degrees) & assists deltoid for 15-90 degrees
Infraspinatus	Infraspinous fossa	Greater tubercle	Suprascapular nerve	Externally rotates
Subscapularis	Subscapular Fossa	Lesser tubercle	Upper and lower subscapular nerves	Internally rotates arm
Teres minor	Lateral border of the scapula	Greater tubercle	Axillary nerve	Externally rotates

References

Drake, R et al. (2010). Gray’s Anatomy for students. 2^ndedition. Philadelphia, PA: Churchill Livingstone Elsevier
Matava MJ, Purcell DB, Rudzki JR (2005). “Partial-thickness rotator cuff tears”. Am J Sports Med. 33 (9): 1405–17. Available from: [LINK].
OpenStax College. Deltoid muscle. Licence: [CC BY]. Available from: [LINK].
Mikael Häggström, used with permission. Teres Major. Available from: [LINK].
Mikael Häggström, used with permission. Supraspinatus. Available from: [LINK].
Mikael Häggström, used with permission. Infraspinatus. Available from: [LINK].
Anatomography. Subscapularis muscle. License [CC BY-SA 2.1 JP]. Available from [LINK].
Anatomography. Teres minor muscle. License [CC BY-SA 2.1 JP]. Available from [LINK].