" Squatting' in a Smith machine is an oxymoron. A Smith machine is not a squat rack, no matter what the girls at the front desk tell you. A squat cannot be performed on a Smith machine any more than it can be performed in a small closet with a hamster. Sorry. There is a gigantic difference between a machine that makes the bar path vertical for you and a squat that is executed correctly enough to have a vertical bar path. The job of keeping the bar path vertical should be done by the muscles, skeleton, and nervous system, not by grease fittings, rails, and floor bolts. "

― Mark Rippetoe , Starting Strength: Basic Barbell Training