The lower trapezius is the most important trapezius component for overhead function, providing the scapular depression and upward rotation forces that prevent the scapula from rising into an elevated, anteriorly tilted position during arm elevation. Its upward rotation contribution via the medial scapular spine forms the most powerful arm of the scapular upward rotation force couple, working with the serratus anterior to ensure the glenoid faces superiorly at full arm elevation.
| Origin | Spinous processes of T4 through T12 |
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| Insertion | Medial apex of the scapular spine (tubercle of the trapezius) |
| Nerve Supply | Spinal accessory nerve (CN XI); Cervical plexus (C3, C4) |
| Blood Supply | Transverse cervical and dorsal scapular arteries |
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As arm elevation increases beyond 90 degrees, the lower trapezius becomes increasingly critical, without its depressing and rotating force, the upper trapezius dominates and the scapula elevates rather than rotating, causing the supraspinatus tendon to impinge against the coracoacromial arch.
Lower trapezius weakness is one of the most consistent findings in scapular dyskinesis with shoulder impingement and rotator cuff disease: reduced upward rotation and posterior tilt of the scapula during overhead activity narrows the subacromial space. Exercises that selectively target it, such as prone Y and T raises and resisted scapular depression and retraction, are a foundation of shoulder impingement rehabilitation. In accessory nerve (CN XI) palsy, for example after neck dissection, all three parts of trapezius are paralysed, producing a drooping shoulder, lateral scapular winging, weak shrug and impaired arm elevation above horizontal.
The lower trapezius is palpable between the lower thoracic spinous processes and the scapular spine triangle, becoming firm during arm elevation with scapular depression (prone Y exercise).
Complete trapezius paralysis from accessory nerve injury during neck dissection producing dropped shoulder, scapular winging, and inability to elevate the arm, managed with nerve repair or shoulder stabilisation.
