Which muscle is assessed with severe downward rotation of the scapulothoracic joint?

The levator scapulae muscle plays a significant role in the downward rotation of the scapula. This muscle originates from the upper cervical vertebrae and inserts onto the superior angle of the scapula. When the levator scapulae contracts, it pulls the scapula upward and medially, which contributes to the downward rotation of the scapula, especially when the arm is elevated or during certain movements.

In contrast, the other muscles listed do not primarily function to cause downward rotation. The trapezius, particularly its lower fibers, generally assists in upward rotation, while the rhomboids are primarily responsible for retraction and elevation rather than downward rotation. The serratus anterior primarily facilitates upward rotation and protraction of the scapula. Therefore, assessing severe downward rotation of the scapulothoracic joint would typically indicate increased activity or tension in the levator scapulae muscle.