Many people like to train on the Smith Machine because it’s safe, it allows for what amounts to a built-in spotter, and it makes it easier to lift heavier loads than you can with free weights. But choosing the Smith Machine might not be best for muscle growth.
In a recent study, Jared Coburn and co-workers from California State University, Northridge found free weight bench presses caused greater activation of the medial deltoid (middle shoulder muscle) than Smith Machine bench presses at 70 and 90 percent of maximal effort (1-rep maximum).
There were no differences in activation levels of the anterior deltoid or pectoralis major (chest) muscles. They measured muscle activation using electromyography. Therefore, free weights cause greater upper body muscle activation during the bench press than the Smith Machine. Also, the Smith Machine requires a specific bar path during exercise, which could place abnormal stresses on the joints and possibly increase the risk of injury.
Source: Journal Strength Conditioning Research, 24: 779-784, 2010