The supine or ‘flat bench’ when compared to an incline bench has functional similarities. Both require horizontal adduction and elbow extension when the lifter initiates and completes the adducting movement. Both exercises also affect the arms humerus position which in turn results in different exercise loads.
The flat bench press develops three portions of the pectoralis major and the anterior deltoid. Then, when an adjustable bench is elevated 30 degrees there is greater activation of the upper portion of the pectoralis major. Once the bench's inclination becomes above 30 degrees there is significantly greater activation of the anterior deltoid, which decreases activity in the three portions of the pectoralis major.
By changing inclination angles, muscle activity and force production impacts neuromuscular adaptation, which is an important training consideration. Over time different elevations lead to greater strength gains than training constantly and consistently one particular chest/shoulder movement.