From Ergo Log
We already knew that it was the case for squats, but it applies to bench presses to. Sports scientists at California State University discovered that you’ll get more out of bench presses if you do them with an old-fashioned barbell rather than on a Smith machine.
In the Journal of Strength and Condition Research, the researchers published the results of an experiment they did with 26 students, of whom 14 had already been doing weight training for at least 6 months, and 12 had no experience in strength sports.
The researchers got their subjects to do 2 reps of bench presses, once with 70 percent and once with 90 percent of their maximal weight [1RM]. The subjects performed the exercise on a Smith machine and on an old-fashioned free-weights bench.
The subjects had electrodes attached to their shoulder and chest muscles so that the researchers could measure how hard the muscles had to work. The results showed that the bench presses done with free weights stimulated nearly all muscle groups more than training on the Smith machine did.
When bench presses are done on the Smith machine it’s the deltoid medialis [MD] that loses out most. the shoulder muscle to the side of your shoulder joint. The second loser is the deltoid anterior [AD], the muscle at the front side of your shoulders. And in third place comes the chest muscle itself, the pectoralis major [PM].
“The strength coach may better serve his/her athletes by choosing the free weight bench over the Smith machine bench press because of its potential for more sports-specific muscular development”, the researchers conclude.
J Strength Cond Res. 2010 Mar; 24(3): 779-84.