Originally posted by Cube:
The Smith machine only permits a one plane of movement, where the weight is stabilized for you. Basically, the shoulders work in more than one plane, infact three. Therefore, they will not be recruited maximally. You will then only be strong in one of these planes of movement and weak in the others. So, in essence you will become more injury prone when you perform a three dimensional bench press. You must use barbells or DBs to allow natural motion of the lift, and to recruit more motor units. Thus, optimum/efficient training stimulus.