When the upper fibers of the trapezius are paralyzed, what happens to the shoulder on that side?

When the upper fibers of the trapezius muscle are paralyzed, the shoulder on that affected side typically presents a lower position compared to the unaffected side. The trapezius muscle plays a crucial role in shoulder elevation and stability. The upper fibers, in particular, are responsible for shrugging the shoulder and raising it towards the ear.

When paralysis occurs, the muscle loses its ability to contract effectively, leading to a lack of support for the shoulder girdle on that side. As a result, the weight of the arm and gravitational forces pull the shoulder downward, resulting in a visually lower position compared to the unaffected shoulder. This phenomenon is commonly observed in conditions that affect the accessory nerve, which innervates the upper trapezius.