The pectoralis major has an insertion point that extends from the lip on the lateral of the bicipital groove to the lip on the anterior side of the deltoid tuberosity. This muscle is responsible for the primary movement of the arm during a dumbbell flat bench chest press.
The anterior deltoid assist the pectoralis major during this exercise. The humerus bone is adducted horizontally, moving it toward the center of the body. This strengthens the arms and chest at the same time. The part of the muscle that attaches at the insertion point is the most movable portion of that muscle.