An
evil piece of exercise
equipment. Ok, maybe not evil... It's basically a large steel
frame with little pegs proturding toward the inside. On each side of the frame, two
guide rails run parallel to the frame from top to bottom. A barbell is attatched to these rails via
bearings. The
barbell also has little hooks on it which can be used to fix the bar to the frame by rotating it so that the hooks catch on the pegs. In addition, there are
typically adjustable pegs which can set in the path of the barbell so that it cannot be dropped below a certain
height.
Now, these things seem like a great idea. First off, they seem pretty safe for lifting without a spotter because you can rack the weight at any point and you can prevent it from dropping below a certain height like say, your chest. In addition, it also gives you smooth movement and the use of free weights.
So why are they bad? The first problem is that it completely eliminates the need to stabilize the weight yourself. Many people may see this as a benefit, as it makes the weight easier to lift, however the use of supporting muscles to stabilize free weights gives you a much better and more thorough workout. Because the machine does not allow the weight to go off course, it is less intense and will not yield as much progress as free weights.
The other reason they are bad is that they force the weight to travel on a fixed path. For exercises such as the bench press where the weight should travel a relatively straight path, this is not so bad. But, for an exercise such as the squat, it can lead to back and knee injury. With free weights, the weight follows a bit of a curve when doing a squat, because this is the path it needs to in order to maintain proper form. So when you do a squat on a Smith Machine, it will force you into improper form, which is a great way to get hurt.
So if you are in a gym, or buying equipment for a home gym, stay away from the Smith Machine. A good power rack will give you just as much safety, a better workout, and will probably be cheaper as well.