Which muscle is NOT involved in shoulder flexion?

The trapezius muscle is primarily responsible for movements of the scapula, including elevation, retraction, and rotation, rather than shoulder flexion. While the upper fibers of the trapezius do assist in elevating the shoulder girdle, they do not directly contribute to the shoulder flexion movement itself, which involves the humerus being moved forward.

In contrast, the biceps brachii, pectoralis major, and coracobrachialis all play significant roles in shoulder flexion. The biceps brachii aids in joint stabilization and assists with shoulder flexion when the elbow is flexed. The pectoralis major can flex the shoulder, especially when the arm is adducted. The coracobrachialis also assists in flexing the arm at the shoulder joint.

Thus, while the muscles that contribute to shoulder flexion include the biceps brachii, pectoralis major, and coracobrachialis, the trapezius does not engage in this particular action, making it the correct choice in the context of this question.