Which of the following muscles is NOT involved in shoulder joint horizontal adduction?

Shoulder joint horizontal adduction involves bringing the arm across the torso, primarily utilizing muscles that help in this movement. The pectoralis major, anterior deltoid, and coracobrachialis all play significant roles in this action.

The pectoralis major is a powerful muscle located in the chest that facilitates horizontal adduction by pulling the arm towards the midline of the body. The anterior deltoid also contributes significantly, as it helps lift the arm forward and assists in bringing it across the body. The coracobrachialis, although less prominent, aids in flexion and adduction of the shoulder, also participating in horizontal adduction.

In contrast, the posterior deltoid is primarily involved in opposing movements, such as horizontal abduction and extension of the shoulder. It functions to move the arm backward rather than across the body. Therefore, when considering which muscles engage during horizontal adduction, the posterior deltoid does not play a supportive role, making it the muscle that is not involved in this action.