The Trapezius is a large, superficial, trapezoid-shaped muscle that covers the upper back and neck. It acts like a coat hanger for the shoulder girdle. Because it is composed of three distinct fiber sets (Upper, Middle, and Lower), it is capable of moving the scapula in multiple directions.
[Image of External Oblique muscle anatomy]Quick Anatomy Snapshot
| Origin (Proximal) | External Occipital Protuberance, Superior Nuchal Line, Nuchal Ligament, and Spinous Processes of C7-T12. |
|---|---|
| Insertion (Distal) |
Upper: Lateral third of Clavicle. Middle: Acromion and Spine of Scapula. Lower: Root of the Spine of the Scapula. |
| Nerve Supply | Spinal Accessory Nerve (Cranial Nerve XI) - Motor. (C3-C4 spinal nerves provide proprioception/pain sensation). |
| Blood Supply | Transverse Cervical Artery. |
| Primary Actions |
|
Deep Dive: Three Muscles in One
Though it is one continuous sheet, the Trapezius functions as three separate muscles depending on which fibers are activated.
1. Upper Fibers (Descending)
These run downward from the skull/neck to the clavicle. They are responsible for shrugging the shoulders (Elevation). In reverse action (when shoulders are fixed), they extend and laterally flex the neck.
2. Middle Fibers (Transverse)
These run horizontally from the upper thoracic spine to the acromion. They are powerful retractors, pulling the shoulder blades together towards the spine.
3. Lower Fibers (Ascending)
These run upward from the lower thoracic spine to the scapular spine. They depress the scapula (pull it down) and assist in upward rotation. Weakness here is a common cause of poor overhead mechanics.
Physio Corner: Clinical Relevance
Palpation
The Trapezius is superficial and easy to feel.
• Upper: Pinch the bulk of the muscle between the neck and shoulder.
• Middle: Palpate between the spine and shoulder blade while the patient squeezes them together.
• Lower: Palpate medial to the scapula (T7-T12 level) while the patient lifts an arm diagonally overhead in prone.
Damage to CN XI (often from lymph node biopsy or trauma in the posterior triangle of the neck) paralyzes the Trapezius. This results in a drooping shoulder, inability to shrug, and difficulty raising the arm above horizontal.
Manual Muscle Testing (MMT)
Because the three parts have different actions, they must be tested separately.
Step-by-Step Procedure (Oxford Scale)
| Part | Patient Action & Resistance |
|---|---|
| Upper (Elevation) |
Position: Sitting. Action: Patient shrugs shoulders toward ears. Resistance: Therapist pushes down on tops of shoulders. |
| Middle (Retraction) |
Position: Prone. Arm abducted 90°, elbow flexed 90°. Action: Patient lifts elbow toward ceiling (squeezes scapula in). Resistance: Applied to lateral scapula/distal humerus pushing down. |
| Lower (Depression) |
Position: Prone. Arm held diagonally overhead (145° abduction) in a "Y" shape, thumb up. Action: Patient lifts arm off the table. Resistance: Applied to distal forearm pushing down. |
Frequently Asked Questions
Why is it called the "Stress Muscle"?
The Upper Trapezius is highly sensitive to emotional stress. People tend to unconsciously hunch or shrug their shoulders when anxious, leading to chronic hypertonicity, trigger points, and tension headaches.
Is it innervated by the spinal cord?
Technically, no. Its motor supply comes from a Cranial Nerve (CN XI), which originates in the brainstem/upper cervical cord but travels distinct from the spinal plexus system. However, it receives sensory input from C3-C4 spinal nerves.
What is the antagonist?
It depends on the fiber:
• Upper Trap antagonist = Lower Trap / Latissimus Dorsi (Depressors).
• Middle Trap antagonist = Serratus Anterior / Pectoralis Minor (Protractors).
Test Your Knowledge: Trapezius Quiz
1. Which nerve provides MOTOR supply to the Trapezius?
2. What is the primary action of the Upper Trapezius?
3. Where does the Trapezius originate superiorly?
4. Which muscle works with the Upper and Lower Trap to produce Upward Rotation?
5. Where does the Lower Trapezius insert?
6. To test the Lower Trapezius, the arm is placed in which position?
7. What is the shape of the Trapezius?
8. Which spinal levels provide the origin for the Trapezius?
9. Paralysis of the Trapezius causes:
10. True or False: The Trapezius is an extrinsic back muscle.
References
- Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
- Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
- Kendall, F. P. (2005). Muscles: Testing and Function, with Posture and Pain. 5th ed. Lippincott Williams & Wilkins.
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