What is the function of the middle and lower fibers of the trapezius?

• A. Stabilize the scapulothoracic joint eccentrically
• B. Elevate the scapula
• C. Protract the scapula
• D. Adduct the humerus

Answer: (A)

The middle and lower fibers work together to keep the scapula stable on the thorax as the arm moves. The middle fibers retract (pull the scapula toward the spine), while the lower fibers depress the scapula and assist in upward rotation. When the arm is raised or when forces act on the shoulder, these fibers typically lengthen under load to control the scapula’s position—this is an eccentric stabilization. That makes stabilization of the scapulothoracic joint the best description of their function. Elevation is handled mainly by the upper fibers, protraction by other muscles (like the serratus anterior), and adduction of the humerus is not a primary role of the trapezius.