Shoulder muscles

Levator scapulae

A muscle that elevates the scapula and assists in the downward rotation of the scapula.

Innervation of Levator scapulae

Dorsal scapular nerve (C5), and occasionally C3, C4 spinal nerves.

Action of Levator scapulae

Elevates the scapula and downwardly rotates the glenoid cavity.

Origin of Levator scapulae

Transverse processes of the first four cervical vertebrae (C1-C4).

Insertion of Levator scapulae

Medial border of the scapula, superior to the scapular spine.

Trapezius

A large muscle that extends down the back, involved in elevation, retraction, and depression of the scapula.

Innervation of Trapezius

Spinal accessory nerve (CN XI) for motor function; C3 and C4 spinal nerves for pain and proprioception.

Action of Trapezius

• Upper fibers: Elevate the scapula and upwardly rotate it.

• Middle fibers: Retract the scapula.

• Lower fibers: Depress the scapula and upwardly rotate it.

Origin of Trapezius

External occipital protuberance, nuchal ligament, medial 1/3 of the superior nuchal line, and spinous processes of C7-T12 vertebrae.

Insertion of Trapezius

Lateral 1/3 of the clavicle, acromion, and spine of the scapula.

Deltoid

A muscle responsible for arm abduction, composed of three distinct sections: anterior, middle, and posterior.

Innervation of Deltoid

Axillary nerve (C5, C6).

Action of Deltoid

• Overall: Abducts the arm past the initial 15^\circ.

• Anterior fibers: Flexion and medial rotation of the arm.

• Middle fibers: Abduction of the arm.

• Posterior fibers: Extension and lateral rotation of the arm.

Origin of Deltoid

Lateral 1/3 of the clavicle, acromion, and spine of the scapula.

Insertion of Deltoid

Deltoid tuberosity of the humerus.

Supraspinatus

A rotator cuff muscle that initiates arm abduction.

Innervation of Supraspinatus

Suprascapular nerve (C5, C6).

Action of Supraspinatus

Initiates arm abduction (first 0^\circ - 15^\circ) and stabilizes the glenohumeral joint.

Origin of Supraspinatus

Supraspinous fossa of the scapula.

Insertion of Supraspinatus

Superior facet of the greater tubercle of the humerus.

Infraspinatus

A rotator cuff muscle that laterally rotates the arm.

Innervation of Infraspinatus

Suprascapular nerve (C5, C6).

Action of Infraspinatus

Laterally rotates the arm and stabilizes the glenohumeral joint.

Origin of Infraspinatus

Infraspinous fossa of the scapula.

Insertion of Infraspinatus

Middle facet of the greater tubercle of the humerus.

Teres major

A muscle that adducts and medially rotates the humerus at the shoulder joint.

Innervation of Teres major

Lower subscapular nerve (C5, C6, C7).

Action of Teres major

Adducts, medially rotates, and extends the arm.

Origin of Teres major

Posterior surface of the inferior angle and lower part of the lateral border of the scapula.

Insertion of Teres major

Medial lip of the intertubercular (bicipital) groove of the humerus.

Subscapularis

A rotator cuff muscle that medially rotates the arm.

Innervation of Subscapularis

Upper and lower subscapular nerves (C5, C6, C7).

Action of Subscapularis

Medially rotates the arm and stabilizes the glenohumeral joint.

Origin of Subscapularis

Subscapular fossa.

Insertion of Subscapularis

Lesser tubercle of the humerus.

Teres minor

A muscle that laterally rotates and adducts the arm.

Innervation of Teres minor

Axillary nerve (C5, C6).

Action of Teres minor

Laterally rotates and adducts the arm, and stabilizes the glenohumeral joint.

Origin of Teres minor

Upper 2/3 of the lateral border of the scapula.

Insertion of Teres minor

Inferior facet of the greater tubercle of the humerus.

Rotator cuff

A group of muscles and their tendons that act to stabilize the shoulder.

Axillary nerve

The nerve that innervates the deltoid and teres minor muscles.

Suprascapular nerve

The nerve that innervates the supraspinatus and infraspinatus muscles.

Quadrangular space

An anatomical space in the shoulder region bordered by the teres minor, teres major, triceps brachii, and surgical neck of the humerus, containing the axillary nerve and posterior circumflex humeral artery.

Triangular space

An anatomical space bordered by the teres minor and major, and the long head of the triceps, containing the scapular circumflex artery.

Triangular interval

Space bordered by teres major and the long and lateral heads of the triceps, containing the deep brachial artery and the radial nerve.

Eccentric contraction

The controlled lowering of a muscle after it has contracted to lift a weight or resistance.

Bicipital groove

The groove on the humerus where the biceps tendon lies, dividing the greater and lesser tubercles.

Glenohumeral joint

The ball-and-socket joint formed between the humerus and the scapula.