Muscle
Trapezius
Superficial back, scapular stabiliser
High-yield
Only muscle innervated by cranial nerve XI (spinal accessory) among the scapular movers; test by shrugging against resistance.
Overview
A large superficial triangular muscle of the upper back and neck that suspends and rotates the scapula. Its upper, middle, and lower fibres act in different directions.
Reference facts
- Origin
Medial third of superior nuchal line, external occipital protuberance, nuchal ligament, and spinous processes of C7–T12
- Insertion
Lateral third of clavicle, acromion, and spine of the scapula
- Action
Elevates (upper), retracts (middle), and depresses (lower) the scapula; upper and lower fibres together rotate the glenoid cavity upward
- Innervation
Spinal accessory nerve (CN XI); proprioception via C3–C4
- Blood supply
Transverse cervical artery (superficial branch)
Clinical correlation
CN XI injury (e.g., posterior triangle surgery) causes shoulder droop and weak overhead abduction.