From the Lab to the Gym: Chest training on the Smith machine.
Although research confirms that the free-weight bench press allows you to use more weight than the Smith machine version of the exercise (due to the unnatural path of the bar on the Smith), we still recommend you make use of the latter because it places greater focus on the pecs.
Researchers from California State University, Fullerton, measured the muscle activity of the middle deltoid, front deltoid and pectoralis major (pecs) when subjects performed bench presses with free weights and on the Smith machine.
As reported in a 2010 issue of the Journal of Strength & Conditioning Research, there was not much difference between the muscle activity of the pecs and front deltoids during either version. However, the muscle activity of the middle deltoid was more than 60% greater on the free-weight bench press than on the Smith machine.
This increased activity was due to the greater stability needed to balance the free-weight barbell as opposed to the fixed path of the Smith machine.
Use the free-weight bench press to go heavier and place more overload on the pec muscles to stimulate greater overall upper-body growth. However, include the Smith machine bench presses to place more emphasis on the pecs, which will also help to stimulate greater growth.