Smith Machines are the devil.
For one thing they completely remove the stabilization aspect of the squat. But go ahead, get up to 300 on a smith machine and then try it with a barbell and see how long you can keep from falling on your ass.
For another thing it restricts the natural range of motion. See each of us has our own biomechanics (the mechanisms that distinctly tie our individual bones and muscles together) and hence we have an individual movement pattern (range of motion) when doing the squat, or anything else for that matter. For example a guy with a long torso and stocky legs will not squat remotely close to how guy with a short torso and long legs does. The smith machine has only one range of motion for either of these individuals because the bar runs on rails in a fixed path (straight up and down or a slight diagonal). Every individual must conform to no matter their distinct movement pattern. This can be devastating to your skeletal system over time.”
“Squatting” in a Smith machine is a oxymoron. A squat cannot be performed on a Smith machine, as should be obvious from all previous discussion. Sorry. There is a gigantic difference between a machine that makes the bar path vertical, and a squat that is excecuted correctly enough to have a vertical bar path. Muscle and skeleton should do the job of keeping the bar path vertical, not grease fittings and floor bolts.– Mark Rippetoe
(Source: startingstrength.com, via barbellsandbatman)