Horizontal adduction refers to the movement of bringing a body part, usually an arm or leg, towards the midline of the body in a horizontal or sideways direction. Full definition
The origins of the pectoralis major at either the sternum or the clavicle and the insertions on the upper arm make ideal for it to perform its primary role of shoulder horizontal adduction (bringing the arms together in front of the body). (strengthandconditioningresearch.com)
The anterior deltoid displays a peak moment arm (length = 16.8 mm) at zero degrees of horizontal adduction (arms to the side) and its lowest moment arm (length = zero) at 140 degrees of shoulder adduction (arm across body). (strengthandconditioningresearch.com)
Thus, it seems to appear that the anterior deltoid displays similar muscle activity between both multi-joint and single - joint exercises involving horizontal adduction. (strengthandconditioningresearch.com)