The trapezius is a large, kite-shaped muscle covering the posterior neck and most of the upper back, with three functionally distinct regions that act in very different directions on the shoulder blade. The upper trapezius elevates and rotates the scapula, the middle fibres pull it toward the spine, and the lower fibres depress and upwardly rotate the glenoid, meaning all three must work in coordinated balance during overhead activities. It is frequently described as the muscle most commonly associated with postural tension headaches and neck pain in sedentary populations.
| Origin | External occipital protuberance, medial nuchal line, nuchal ligament, and spinous process of C7; Spinous processes of T1–T5; Spinous processes of T6–T12 |
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| Insertion | Posterior border of the lateral third of the clavicle; Medial border of the acromion and superior lip of the spine of the scapula; Medial end of the scapular spine |
| Nerve Supply | Accessory nerve (CN XI), motor; Ventral rami of C3 and C4, proprioception and some pain |
| Blood Supply | Transverse cervical artery; Dorsal scapular artery |
| Actions | Elevates the scapula, retracts the scapula, and upwardly rotates the glenoid; Retracts the scapula; Depresses the scapula and upwardly rotates the glenoid |
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The lower and upper trapezius must act as a force couple with the serratus anterior to produce clean upward rotation of the scapula during arm elevation, weakness or inhibition of any component disrupts this coupling and leads to compensatory patterns involving the levator scapulae and upper trapezius overworking.
Upper trapezius overactivity and myofascial trigger points are among the most common findings in desk workers and overhead athletes, producing a characteristic referral pattern of pain up the neck and into the temple and eye. Lower trapezius weakness is a hallmark finding in shoulder impingement syndrome, as it fails to adequately depress the scapula and upwardly rotate the glenoid during arm raising, reducing subacromial space. The accessory nerve's relatively superficial course through the posterior triangle of the neck makes it vulnerable to iatrogenic injury during neck dissection surgery.
The upper trapezius is easily palpable as the fleshy ridge running from the base of the skull to the tip of the shoulder, it tenses during shrugging and is often tender in people with chronic neck tension. The lower trapezius is best felt medial to the scapula in the lower thoracic region during resisted scapular depression.
Chronic overload or sustained contraction of the upper trapezius produces trigger points with characteristic referral into the neck, temple, and behind the eye, among the most common sources of tension-type headache and neck pain in sedentary workers.
Damage to the spinal accessory nerve from neck surgery or trauma causes weakness of the entire trapezius, resulting in shoulder drooping, winging of the medial scapular border, and difficulty raising the arm above horizontal.
Acute muscle fibre tears from sudden neck or shoulder loading, such as catching a heavy fall, produce localised pain between the neck and shoulder with restricted neck rotation and shoulder elevation.
