Final answer:
Shoulder horizontal abduction and adduction occur in the coronal plane, involving movements of the arm away from and toward the body's midline, respectively.
Step-by-step explanation:
The plane in which shoulder horizontal abduction and adduction occur is the coronal plane, also known as the frontal plane. This plane divides the body into anterior (front) and posterior (back) portions, with movements occurring parallel to this plane. Shoulder horizontal abduction is the movement of the arm away from the midline of the body, which involves the shoulder muscles such as the deltoid. In contrast, shoulder horizontal adduction brings the arm toward the midline, engaging muscles like the pectoralis major.
Both movements are essential for various activities and exercises that involve the upper limbs. It is important to understand these motions to prevent injuries that can occur with activities that repetitively use these movements, like swimming or throwing.