M01.03.018 Shoulder Region:The Extrinsic Muscles of the Shoulder

Written by Oliver Jones

The shoulder muscles are essential for upper limb movement and shaping the shoulder. They are categorized into two main groups:

Extrinsic Muscles: Originate from the torso and attach to the clavicle, scapula, or humerus.
Intrinsic Muscles: Originate from the scapula or clavicle and attach to the humerus.

Note: Additional muscles, such as those in the pectoral region and upper arm, also act on the shoulder joint.

This guide focuses on the extrinsic muscles of the shoulder, covering their anatomy, attachments, innervation, and actions.

Extrinsic Muscles of the Shoulder

Extrinsic muscles are located in the back and categorized into two layers:

Superficial Layer: Includes the trapezius and latissimus dorsi.
Deep Layer: Includes the levator scapulae and rhomboid muscles (major and minor).

Muscle	Attachments	Actions	Innervation
Trapezius	– Origin: Skull, nuchal ligament, spinous processes of C7-T12. – Insertion: Clavicle, acromion, scapula spine.	– Elevates and rotates scapula (upper fibers). – Retracts scapula (middle fibers). – Depresses scapula (lower fibers).	Accessory nerve (motor); C3, C4 (proprioception).
Latissimus Dorsi	– Origin: Spinous processes of T7-T12, iliac crest, thoracolumbar fascia, and inferior three ribs. – Insertion: Intertubercular sulcus of humerus.	– Extends, adducts, and medially rotates upper limb.	Thoracodorsal nerve

Muscle	Attachments	Actions	Innervation
Levator Scapulae	– Origin: Transverse processes of C1-C4. – Insertion: Medial border of scapula.	Elevates scapula.	Dorsal scapular nerve.
Rhomboid Major	– Origin: Spinous processes of T2-T5. – Insertion: Medial border of scapula (inferior angle).	Retracts and rotates scapula.	Dorsal scapular nerve.
Rhomboid Minor	– Origin: Spinous processes of C7-T1. – Insertion: Medial border of scapula (level of spine).	Retracts and rotates scapula.	Dorsal scapular nerve.

Iatrogenic Injuries: Often due to cervical lymph node biopsy or cannulation of the internal jugular vein.

Method: Ask the patient to shrug their shoulders.
Signs of Damage:
- Muscle wasting.
- Partial paralysis of the sternocleidomastoid.
- Asymmetrical neckline.

Extrinsic muscles of the shoulder are organized into superficial (trapezius, latissimus dorsi) and deep (levator scapulae, rhomboids) layers.
Innervation is critical for proper shoulder movement:
- Trapezius: Accessory nerve.
- Latissimus dorsi: Thoracodorsal nerve.
- Levator scapulae and rhomboids: Dorsal scapular nerve.
Clinical testing, such as shoulder shrug tests, can help diagnose nerve damage.

Layer	Muscles	Key Functions
Superficial	Trapezius, Latissimus Dorsi	Elevation, rotation, retraction, extension, adduction.
Deep	Levator Scapulae, Rhomboids	Elevation, retraction, rotation of scapula.