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 |
|---|---|
| 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 |
| Actions | Depression of the scapula; Upward rotation of the scapula (primary action with serratus anterior); Retraction of the scapula |
|---|
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 atrophy is identifiable as the loss of the normal lower scapular muscular contour in the lower thoracic region, producing the inferior scapular winging distinct from the medial border winging of serratus anterior palsy. In CN XI palsy all three trapezius portions are paralysed, producing the characteristic drooped shoulder with inability to shrug, weak scapular retraction, and impaired arm elevation above horizontal. Lower trapezius strengthening is the exercise most strongly associated with reduced shoulder impingement symptoms.
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.
