Six Exercises You Should NEVER Do-#6

The Smith Machine

The myth: This machine -- which looks like a squat rack with a built-in bar that runs on guides-gives you all the benefits of squats, but none of the risk that comes from holding a heavy barbell across your back. That's because the bar can easily be secured at any point during the movement.

The truth: Because the bar runs on guides, you can only move straight up and down as you squat-instead of down and back, as you would in a free-weight squat.

The result: An unnatural movement that puts extra stress on your knees and lower back. Need another reason to skip the Smith? Canadian researchers found that traditional squats produced almost 50 percent more muscle activity in the quadriceps than squats done on a Smith machine.

The alternative: If you're not comfortable with barbell squats, simply do the exercise while holding dumbbells at arm's length next to your sides. You won't need a spotter, and your body will be free to move through the natural motion of the squat.