Machine squats may limit your gains. Canadian scientists say free weights work your muscles harder than the Smith machine, which is the squat station with a built-in bar that runs on guides. “The Smith machine stabilizes the barbell, requiring your muscles to work less,” says study author Phil Chilibeck, Ph.D. Free weights, like dumbbells or barbells, provide a better overall training effect by stressing more muscle fibers in your body. In fact, there is a 43% increase in leg-muscle activation doing free-weight squats instead of using a Smith machine.

Source: Muscle Bulletin; Men’s Health Magazine, March 2010