Learning Objectives
By the end of this lesson, the student should be able to:
- Classify the muscles of the shoulder into extrinsic and intrinsic groups.
- Describe the attachments, actions, and innervation of the extrinsic shoulder muscles.
- Differentiate between the superficial and deep layers of the extrinsic group.
- Apply anatomical knowledge to clinical scenarios involving nerve injury.
Overview of the Extrinsic Muscles of the Shoulder
The muscles of the shoulder enable movement of the upper limb and shape its contour. They are divided into:
Extrinsic Muscles
- Originating from the torso
- Attach to the clavicle, scapula, or humerus
Intrinsic Muscles
- Originates from the scapula and/or clavicle
- Attach to the humerus
This section focuses on the extrinsic muscles of the shoulder, also known as the superficial back muscles. These muscles originate from the trunk and attach to the bones of the shoulder.
They are arranged in two layers:
- Superficial layer
- Deep layer
Superficial Layer
Trapezius
The trapezius is a broad, flat, triangular muscle that forms a trapezoid shape across the upper back.
| Feature | Description |
|---|---|
| Attachments | Skull, nuchal ligament, spinous processes of C7–T12 → clavicle, acromion, and spine of scapula |
| Actions | |
| • Upper fibers: elevate and rotate the scapula | |
| • Middle fibers: retract scapula | |
| • Lower fibers: depress scapula | |
| Innervation | Accessory nerve (motor), C3–C4 (proprioception) |
Clinical Relevance – Testing the Accessory Nerve
Damage to the accessory nerve is most commonly iatrogenic, particularly after cervical lymph node biopsy or internal jugular vein cannulation.
Test: Ask the patient to shrug their shoulders against resistance.
Findings: Shoulder droop, muscle wasting, SCM weakness, asymmetrical neckline.
Latissimus Dorsi
The latissimus dorsi is a large, flat muscle that covers the lower back.
| Feature | Description |
|---|---|
| Attachments | T7–T12 spinous processes, iliac crest, thoracolumbar fascia, ribs 10–12 → intertubercular sulcus of humerus |
| Actions | Extension, adduction, and medial rotation of the arm |
| Innervation | Thoracodorsal nerve |
Deep Layer
Located beneath the trapezius, this layer consists of the levator scapulae and the rhomboids.
Levator Scapulae
| Feature | Description |
|---|---|
| Attachments | Transverse processes of C1–C4 → medial border of scapula |
| Action | Elevates scapula |
| Innervation | Dorsal scapular nerve |
Rhomboids
Two muscles: rhomboid major and rhomboid minor
| Muscle | Attachments | Actions | Innervation |
|---|---|---|---|
| Rhomboid Major | T2–T5 spinous processes → medial border of scapula (below spine) | Retracts & rotates scapula | Dorsal scapular nerve |
| Rhomboid Minor | C7–T1 spinous processes → medial border at spine of scapula | Retracts & rotates the scapula | Dorsal scapular nerve |
