Shoulder Muscles

The shoulder muscles form a coordinated muscular system that positions the scapula, stabilizes the glenohumeral joint, and moves the upper limb through a large range of motion. Because movement of the arm depends on both the glenohumeral joint and movement of the scapula on the thoracic wall, shoulder function requires coordinated activity across several groups of muscles.

The intrinsic muscles of the shoulder include the deltoid, supraspinatus, infraspinatus, teres minor, subscapularis, and teres major. The supraspinatus, infraspinatus, teres minor, and subscapularis form the rotator cuff.

Other muscles, including trapezius, serratus anterior, levator scapulae, rhomboid major, rhomboid minor, pectoralis minor, and subclavius, position the pectoral girdle and indirectly influence movements of the shoulder joint.

Functional Groups of Shoulder Muscles

Group	Major Muscles	Principal Function
Intrinsic shoulder muscles	Deltoid, teres major	Movement of humerus
Rotator cuff	Supraspinatus, infraspinatus, teres minor, subscapularis	Glenohumeral stabilization and rotation
Scapular stabilizers and movers	Trapezius, serratus anterior, rhomboids, levator scapulae, pectoralis minor	Position and rotate scapula

Deltoid

The deltoid is the large triangular muscle forming the rounded contour of the shoulder. It covers the glenohumeral joint and proximal humerus and is one of the principal muscles responsible for elevation of the arm.

Feature	Deltoid
Origin	Lateral third of clavicle, acromion, and spine of scapula
Insertion	Deltoid tuberosity of humerus
Innervation	Axillary nerve
Primary action	Abduction of arm

Parts of the Deltoid

The deltoid can be functionally divided into anterior, middle, and posterior fibers. Their different orientations allow the muscle to participate in several movements of the glenohumeral joint.

Part	Major Actions
Anterior fibers	Flex and medially rotate arm
Middle fibers	Abduct arm
Posterior fibers	Extend and laterally rotate arm

Deltoid and Arm Abduction

The deltoid is the major muscle responsible for continued abduction of the arm after the movement has been initiated. Its middle fibers have a particularly favorable line of pull for elevating the humerus.

Supraspinatus assists with initiation of abduction and remains active during the movement, while upward rotation of the scapula by trapezius and serratus anterior permits elevation of the upper limb beyond the range available at the glenohumeral joint alone.

Axillary Nerve

The axillary nerve arises from the posterior cord of the brachial plexus and supplies the deltoid and teres minor. It passes through the quadrangular space with the posterior circumflex humeral vessels and winds around the surgical neck of the humerus.

This close relationship makes the nerve vulnerable in injuries involving the surgical neck of the humerus and in some shoulder dislocations.

Rotator Cuff

The rotator cuff is formed by the tendons of four scapulohumeral muscles: supraspinatus, infraspinatus, teres minor, and subscapularis. Their tendons blend with the fibrous capsule of the glenohumeral joint.

The rotator cuff provides dynamic stabilization by compressing the humeral head against the glenoid cavity during movement.

Muscle	Insertion	Major Action	Innervation
Supraspinatus	Greater tubercle	Initiates and assists abduction	Suprascapular nerve
Infraspinatus	Greater tubercle	Lateral rotation	Suprascapular nerve
Teres minor	Greater tubercle	Lateral rotation	Axillary nerve
Subscapularis	Lesser tubercle	Medial rotation	Upper and lower subscapular nerves

Supraspinatus

The supraspinatus occupies the supraspinous fossa of the scapula. Its tendon passes beneath the acromion toward the superior facet of the greater tubercle of the humerus.

Feature	Supraspinatus
Origin	Supraspinous fossa of scapula
Insertion	Superior facet of greater tubercle of humerus
Action	Initiates and assists abduction; stabilizes humeral head
Innervation	Suprascapular nerve

Supraspinatus and Abduction

Supraspinatus contributes strongly during the initial phase of arm abduction and works with the deltoid throughout the movement. It also stabilizes the humeral head against the glenoid cavity as the deltoid generates superiorly directed force.

Infraspinatus

The infraspinatus occupies much of the infraspinous fossa on the posterior surface of the scapula.

Feature	Infraspinatus
Origin	Infraspinous fossa of scapula
Insertion	Middle facet of greater tubercle of humerus
Action	Laterally rotates arm and stabilizes glenohumeral joint
Innervation	Suprascapular nerve

Teres Minor

The teres minor is a narrow muscle along the lateral border of the scapula. It is one of the posterior components of the rotator cuff.

Feature	Teres Minor
Origin	Middle portion of lateral border of scapula
Insertion	Inferior facet of greater tubercle of humerus
Action	Laterally rotates arm and assists adduction
Innervation	Axillary nerve

Subscapularis

The subscapularis is a large triangular muscle covering the anterior surface of the scapula. It forms the anterior component of the rotator cuff.

Feature	Subscapularis
Origin	Subscapular fossa
Insertion	Lesser tubercle of humerus
Action	Medially rotates and adducts arm; stabilizes humeral head
Innervation	Upper and lower subscapular nerves

SITS Muscles

The four rotator cuff muscles are commonly remembered as the SITS muscles: supraspinatus, infraspinatus, teres minor, and subscapularis.

Rotator Cuff Attachments

Three rotator cuff muscles insert onto the greater tubercle of the humerus. Supraspinatus inserts on the superior facet, infraspinatus on the middle facet, and teres minor on the inferior facet. Subscapularis inserts onto the lesser tubercle.

Dynamic Stabilization of the Glenohumeral Joint

The glenoid cavity is relatively shallow compared with the large humeral head. This arrangement permits extensive mobility but provides limited inherent bony stability.

The rotator cuff muscles help compensate by maintaining compression of the humeral head against the glenoid during movement. Their coordinated activity helps prevent excessive translation of the humeral head.

Teres Major

The teres major is a thick muscle extending from the inferior scapula to the proximal humerus. Despite its name and proximity to teres minor, it is not part of the rotator cuff.

Feature	Teres Major
Origin	Posterior surface near inferior angle of scapula
Insertion	Medial lip of intertubercular sulcus of humerus
Action	Adducts and medially rotates arm; assists extension
Innervation	Lower subscapular nerve

Teres Major and Latissimus Dorsi

Teres major and latissimus dorsi have similar actions at the glenohumeral joint. Both contribute to extension, adduction, and medial rotation of the humerus.

Scapular Muscles

Normal shoulder movement requires precise positioning of the scapula. The major muscles responsible for scapular movement include trapezius, serratus anterior, rhomboid major, rhomboid minor, levator scapulae, and pectoralis minor.

Trapezius

The trapezius is a large superficial muscle extending from the skull and vertebral column to the clavicle and scapula. Its upper, middle, and lower fibers perform different but coordinated actions.

Part	Major Action
Upper fibers	Elevate scapula and assist upward rotation
Middle fibers	Retract scapula
Lower fibers	Depress scapula and assist upward rotation

Trapezius Attachments

Trapezius arises broadly from the superior nuchal region, external occipital protuberance, nuchal ligament, and spinous processes of the cervical and thoracic vertebrae. It inserts onto the lateral clavicle, acromion, and spine of the scapula.

Innervation of Trapezius

The motor supply of trapezius is provided primarily by the spinal accessory nerve. Cervical spinal nerves provide proprioceptive contributions.

Serratus Anterior

The serratus anterior extends from the upper ribs to the anterior surface of the medial border of the scapula. It holds the scapula against the thoracic wall and is a major contributor to scapular protraction and upward rotation.

Feature	Serratus Anterior
Origin	External surfaces of upper eight or nine ribs
Insertion	Anterior surface of medial border of scapula
Actions	Protracts scapula, holds it against thoracic wall, assists upward rotation
Innervation	Long thoracic nerve

Long Thoracic Nerve

The long thoracic nerve descends superficially along serratus anterior and supplies the muscle. Its relatively superficial course makes it susceptible to injury during trauma or procedures involving the lateral thoracic wall and axillary region.

Winged Scapula

Weakness or paralysis of serratus anterior can produce a winged scapula, in which the medial border of the scapula becomes unusually prominent away from the thoracic wall.

Scapular upward rotation may also be impaired, making elevation of the upper limb more difficult.

Rhomboid Major

Feature	Rhomboid Major
Origin	Spinous processes of upper thoracic vertebrae
Insertion	Medial border of scapula below level of spine
Actions	Retracts and downwardly rotates scapula; stabilizes scapula
Innervation	Dorsal scapular nerve

Rhomboid Minor

Feature	Rhomboid Minor
Origin	Nuchal ligament and lower cervical to upper thoracic spinous processes
Insertion	Medial end of spine of scapula
Actions	Retracts and stabilizes scapula; assists downward rotation
Innervation	Dorsal scapular nerve

Levator Scapulae

The levator scapulae connects the upper cervical vertebrae with the superior medial scapula.

Feature	Levator Scapulae
Origin	Transverse processes of upper cervical vertebrae
Insertion	Medial border of scapula superior to spine
Actions	Elevates scapula and assists downward rotation
Innervation	Dorsal scapular nerve and cervical nerves

Pectoralis Minor

The pectoralis minor lies deep to pectoralis major and connects the ribs to the coracoid process.

Feature	Pectoralis Minor
Origin	Anterior surfaces of ribs 3 to 5 near their costal cartilages
Insertion	Coracoid process of scapula
Actions	Stabilizes scapula by drawing it anteriorly and inferiorly
Innervation	Medial pectoral nerve

Subclavius

The subclavius is a small muscle extending from the first rib to the inferior surface of the clavicle. It anchors and depresses the clavicle and contributes to stability of the sternoclavicular region.

Feature	Subclavius
Origin	First rib and costal cartilage
Insertion	Inferior surface of middle third of clavicle
Action	Anchors and depresses clavicle
Innervation	Nerve to subclavius

Scapular Elevation

Elevation of the scapula is produced principally by the upper fibers of trapezius and levator scapulae, with contributions from the rhomboids.

Scapular Depression

Scapular depression involves the lower fibers of trapezius and other muscles capable of drawing the shoulder girdle inferiorly. The exact muscular contribution varies with the position and loading of the upper limb.

Scapular Protraction

Serratus anterior is the major protractor of the scapula. Pectoralis minor can assist by drawing the coracoid process anteriorly.

Protraction is important during pushing and punching movements.

Scapular Retraction

Retraction draws the scapula medially toward the vertebral column. The middle fibers of trapezius and the rhomboids are major contributors.

Upward Rotation of the Scapula

Upward rotation turns the glenoid cavity superiorly and is essential for full elevation of the arm. It is produced by a force couple involving serratus anterior and the upper and lower fibers of trapezius.

Downward Rotation of the Scapula

Downward rotation returns the glenoid cavity toward its resting orientation. The rhomboids and levator scapulae contribute to this movement, with assistance from other muscles depending on the position of the limb.

Scapulohumeral Rhythm

Scapulohumeral rhythm describes the coordinated relationship between movement at the glenohumeral joint and movement of the scapula during elevation of the upper limb.

After the initial phase of elevation, glenohumeral movement and scapular rotation occur together. A commonly described overall relationship is approximately 2:1, meaning that roughly two degrees of glenohumeral movement accompany one degree of scapular rotation, although the ratio varies through the range of motion.

Arm Flexion

Flexion at the shoulder is produced principally by the anterior deltoid and clavicular portion of pectoralis major, with contributions from coracobrachialis and biceps brachii.

Arm Extension

Extension of the arm is produced by muscles including the posterior deltoid, latissimus dorsi, and teres major. The long head of triceps brachii can also assist.

Arm Abduction

Abduction requires coordinated activity of supraspinatus and deltoid at the glenohumeral joint and trapezius and serratus anterior for upward rotation of the scapula.

Arm Adduction

Powerful adduction is produced by muscles including pectoralis major, latissimus dorsi, and teres major. Coracobrachialis and the long head of triceps can contribute.

Medial Rotation

Medial rotation of the humerus is produced by several muscles, including subscapularis, pectoralis major, latissimus dorsi, teres major, and anterior deltoid.

Lateral Rotation

The principal lateral rotators of the humerus are infraspinatus and teres minor, with assistance from the posterior fibers of the deltoid.

Shoulder Muscle Actions

Movement	Major Muscles
Abduction	Supraspinatus, deltoid
Adduction	Pectoralis major, latissimus dorsi, teres major
Flexion	Anterior deltoid, clavicular pectoralis major, coracobrachialis
Extension	Posterior deltoid, latissimus dorsi, teres major
Medial rotation	Subscapularis, pectoralis major, latissimus dorsi, teres major, anterior deltoid
Lateral rotation	Infraspinatus, teres minor, posterior deltoid

Quadrangular Space

The quadrangular space is an important passage in the posterior shoulder. It transmits the axillary nerve and posterior circumflex humeral vessels.

Its boundaries include teres minor superiorly, teres major inferiorly, the long head of triceps medially, and the surgical neck of the humerus laterally.

Triangular Space

The triangular space is bounded by teres minor superiorly, teres major inferiorly, and the long head of triceps laterally. It transmits the circumflex scapular vessels.

Triangular Interval

The triangular interval lies inferior to teres major and is associated with passage of the radial nerve and profunda brachii vessels toward the posterior arm.

Subacromial Region

The supraspinatus tendon passes through the confined region beneath the acromion and coracoacromial arch. The subacromial-subdeltoid bursa reduces friction between the supraspinatus tendon, deltoid, and overlying structures during shoulder movement.

Rotator Cuff Tendinopathy

Rotator cuff tendinopathy commonly affects the supraspinatus tendon, although other cuff tendons may also be involved. Repetitive loading, degeneration, trauma, and altered shoulder mechanics can contribute to tendon pathology.

Rotator Cuff Tears

A rotator cuff tear may be partial or full thickness and can involve one or multiple tendons. Supraspinatus is frequently involved because of its anatomy and loading during arm elevation.

Functional consequences depend on the size and location of the tear and may include weakness of abduction or rotation and impaired dynamic stabilization of the glenohumeral joint.

Suprascapular Nerve

The suprascapular nerve arises from the upper trunk of the brachial plexus and supplies supraspinatus and infraspinatus. It passes toward the suprascapular notch and then around the scapula to reach the infraspinous region.

Suprascapular Nerve Injury

Suprascapular nerve dysfunction can weaken shoulder abduction through supraspinatus involvement and lateral rotation through infraspinatus involvement. The pattern depends on the level of nerve injury.

Axillary Nerve Injury

Axillary nerve injury can weaken the deltoid and teres minor. Significant deltoid weakness impairs arm abduction, particularly after the initial phase of the movement.

Atrophy of the deltoid may reduce the normal rounded contour of the shoulder.

Shoulder Dislocation and the Axillary Nerve

The axillary nerve lies close to the inferior aspect of the glenohumeral joint and surgical neck of the humerus. It can therefore be affected by some glenohumeral dislocations and proximal humeral injuries.

Testing the Deltoid

The deltoid can be assessed by asking the individual to abduct the arm against resistance. The muscle can be observed and palpated over the lateral shoulder.

Testing Supraspinatus

Supraspinatus function can be assessed during resisted abduction in a position that emphasizes the supraspinatus and rotator cuff. Interpretation should consider that the deltoid also contributes substantially to abduction.

Testing Infraspinatus

Infraspinatus can be assessed by asking the individual to laterally rotate the arm against resistance while the elbow is flexed.

Testing Teres Minor

Teres minor contributes to resisted lateral rotation. Because infraspinatus performs a similar action, clinical testing often evaluates the lateral rotator group rather than completely isolating teres minor.

Testing Subscapularis

Subscapularis function can be evaluated through resisted medial rotation. Clinical maneuvers may place the hand behind the back or against the abdomen to assess the ability to generate internal rotation force.

Testing Serratus Anterior

Serratus anterior can be assessed by asking the individual to push against a wall while the scapulae are observed. Weakness may allow prominence of the medial border of the scapula.

Testing Trapezius

Upper trapezius can be tested through resisted shoulder elevation. Middle fibers can be assessed during resisted scapular retraction, while lower fibers can be evaluated during movements involving scapular depression and upward rotation.

Testing the Rhomboids

The rhomboids can be assessed through resisted scapular retraction and downward rotation. Their function should be distinguished from the retraction produced by the middle fibers of trapezius.

Shoulder Stability

Shoulder stability depends on a combination of static and dynamic structures. Static stabilizers include the glenoid labrum, joint capsule, ligaments, and articular geometry. Dynamic stabilization is provided largely by the rotator cuff and other muscles crossing or controlling the shoulder complex.

Force Couples at the Shoulder

A force couple occurs when muscles generate forces in different directions that combine to produce controlled rotation or stabilization. Shoulder function relies heavily on these coordinated muscular relationships.

Trapezius and serratus anterior form an important force couple for upward rotation of the scapula, while the rotator cuff balances forces generated by the deltoid around the glenohumeral joint.

Deltoid and Rotator Cuff Force Balance

During arm elevation, the deltoid generates a strong superior component of force on the humerus. The rotator cuff contributes compressive and stabilizing forces that help maintain the humeral head in an appropriate relationship with the glenoid.

This coordinated action allows effective elevation without excessive superior translation of the humeral head.

Scapular Stabilization During Upper Limb Movement

The scapula provides the mobile base from which many shoulder and arm muscles act. Stabilization by trapezius, serratus anterior, rhomboids, and other muscles is therefore essential for efficient generation and transmission of force through the upper limb.

Overhead Movement

Raising the arm overhead requires glenohumeral elevation, scapular upward rotation, clavicular movement, and coordinated stabilization. Deltoid and supraspinatus elevate the humerus, while trapezius and serratus anterior rotate the scapula upward.

Pushing Movements

Pushing requires stabilization and protraction of the scapula, particularly through serratus anterior. Depending on the movement, pectoral muscles, triceps, and other upper limb muscles contribute to generating force.

Pulling Movements

Pulling commonly requires scapular retraction through trapezius and rhomboids, combined with shoulder extension or adduction through muscles such as latissimus dorsi, posterior deltoid, and teres major.

Shoulder Muscle Innervation Summary

Muscle	Principal Innervation
Deltoid	Axillary nerve
Supraspinatus	Suprascapular nerve
Infraspinatus	Suprascapular nerve
Teres minor	Axillary nerve
Subscapularis	Upper and lower subscapular nerves
Teres major	Lower subscapular nerve
Trapezius	Spinal accessory nerve
Serratus anterior	Long thoracic nerve
Rhomboid major and minor	Dorsal scapular nerve
Levator scapulae	Dorsal scapular and cervical nerves
Pectoralis minor	Medial pectoral nerve
Subclavius	Nerve to subclavius

Blood Supply

The shoulder muscles receive blood through an extensive arterial network derived primarily from branches of the subclavian and axillary arteries. Important vessels include the suprascapular, dorsal scapular, circumflex scapular, thoracoacromial, lateral thoracic, and anterior and posterior circumflex humeral arteries.

Scapular Anastomosis

The scapular arterial anastomosis forms a network around the scapula connecting branches of the subclavian and axillary arterial systems. Important contributors include the suprascapular, dorsal scapular, and circumflex scapular arteries.

Circumflex Humeral Arteries

The anterior and posterior circumflex humeral arteries pass around the surgical neck region of the humerus. The posterior circumflex humeral artery accompanies the axillary nerve through the quadrangular space and contributes significantly to the blood supply of the deltoid and shoulder region.

Summary of Major Shoulder Muscles

Muscle	Major Action	Innervation
Deltoid	Arm abduction; fibers also flex, extend and rotate arm	Axillary nerve
Supraspinatus	Initiates and assists abduction	Suprascapular nerve
Infraspinatus	Lateral rotation	Suprascapular nerve
Teres minor	Lateral rotation	Axillary nerve
Subscapularis	Medial rotation	Subscapular nerves
Teres major	Adduction, medial rotation and extension	Lower subscapular nerve
Trapezius	Elevates, retracts, depresses and upwardly rotates scapula	Spinal accessory nerve
Serratus anterior	Protraction and upward rotation	Long thoracic nerve
Rhomboids	Retraction and downward rotation	Dorsal scapular nerve
Levator scapulae	Scapular elevation and downward rotation	Dorsal scapular and cervical nerves

Key Anatomical Features

Feature	Key Point
Major abductor	Deltoid
Abduction initiator and stabilizer	Supraspinatus
Rotator cuff	Supraspinatus, infraspinatus, teres minor, subscapularis
Main lateral rotators	Infraspinatus and teres minor
Rotator cuff medial rotator	Subscapularis
Major scapular protractor	Serratus anterior
Major scapular retractors	Middle trapezius and rhomboids
Upward rotation force couple	Trapezius and serratus anterior
Deltoid nerve	Axillary nerve
Serratus anterior nerve	Long thoracic nerve
Trapezius motor nerve	Spinal accessory nerve
Primary functional principle	Coordination of glenohumeral movement, scapular movement and dynamic stabilization

Anatomical and Functional Importance

The shoulder muscles operate as an integrated system rather than as isolated movers. The exceptional mobility of the glenohumeral joint requires continuous muscular stabilization, particularly from the rotator cuff, while movement of the scapula provides a changing platform for the humerus.

The deltoid generates much of the force required for arm elevation, while the rotator cuff maintains the relationship between the humeral head and glenoid cavity. Supraspinatus assists abduction, infraspinatus and teres minor provide lateral rotation, and subscapularis provides medial rotation while all four muscles contribute to stability.

At the same time, trapezius and serratus anterior rotate the scapula upward during overhead movement, while rhomboids, levator scapulae, pectoralis minor, and other muscles control its position in response to functional demands.

Understanding the shoulder muscles is therefore fundamental to the anatomy of arm elevation, glenohumeral stability, scapulohumeral rhythm, rotator cuff function, shoulder rotation, scapular movement, axillary and suprascapular nerve lesions, winged scapula, and common shoulder injuries.