The squat, lunge, bench press and overhead press are all great exercises--they are compound movements that will help improve strength, power, and stability. Unfortunately, I see many people performing this movement on a machine that limits the benefits of these movements: the Smith machine.
|Don't try this at home...or anywhere else, for that matter.|
You've probably seen this machine at your gym before. The Smith machine is where great exercises go to die.
The reason why I am so adamantly against the usage of the Smith machine is simple: the machine creates stability for you by forcing you to move through a fixed range of motion. Part of the glory of a squat or bench press is that our bodies must find stability on their own. When you take a barbell out of the rack, you may feel a little bit shaky at first. Your shoulder stabilizers are working hard to prevent the barbell from yanking your arms behind you during the bench press. The same phenomenon occurs on the squat. If you can't properly stabilize your hips, you will either fall backwards or forwards, or your knees will cave inwards as you stand.
Squatting on the Smith machine is basically like telling your core to go take a nap while you do your set. You will notice that you will be able to squat significantly more weight on a Smith machine, because you don't need to stabilize your midline! As a result, your legs can concentrate solely on moving the load on your back. While this may sound like a positive, I can assure you it is not. If you want to improve your body's reflexive strength and prevent unnecessary injuries like rolled ankles, then stick to free weights.
When you're walking down the street, you are not moving on a fixed plane. Your hip adductors work to prevent excessive hip abduction, and vice versa. Your torso must rotate slightly to navigate your center of mass. These actions are innate and help create efficient movement. Similar reactions happen in the squat: your abductors prevent the ugly valgus knee position, your erector spinae instinctively push against the barbell to prevent your torso upright, and your glutes are suppose to help you extend the hips as you stand. Many of these muscles will go to sleep when you squat on the Smith machine, because they aren't necessary.
Don't cheat yourself. Wake up your stabilizer muscles and train with purpose.