There's a Smith Machine in virtually every commercial gym in the Western World. Some consider this a sign that the Smith Machine is a worthwhile and valuable piece of exercise equipment. This is not the case. Remember those vibrating belt machines from the 1950's or so? They were in every gym in the US, and they did absolutely nothing but make you jiggle while standing still. (See picture at the bottom of this post.)
"But wait," you say, "I've heard that the Smith Machine is a great way to do Squats, Bench Press, and other exercises when you don't have a spotter."
NOT TRUE. Like any machine, the Smith Machine is inferior to free weights in many ways:
Many people think that the Smith Machine is a safer alternative to barbell exercises, but this is not the case. The Smith Machine might feel safer, but the fixed bar path actually makes many exercises more dangerous.
The most obvious example is the Bench Press. With free weights, there are a number of ways to get out of a failed rep without a spotter. In a worst-case scenario, when the bar is on your chest and you absolutely cannot lift it off, you could lean to one side until the weights fell off to free yourself. If the same situation arose in a Smith Machine, the bar is held in place, so your only option is to scream for help and/or wait to die as the weight slowly crushes you. Your call.
Some people think Squatting is safer in the Smith Machine, as well. This guy probably used to think so, too:
This wouldn't have happened with free weights, because he would've felt his balance shifting when the weight was too far forward and compensated by taking a step forward.
The Smith Machine's primary characteristic is that it removes the need to stabilize the weight while lifting. This means that the stabilizing muscles that you would normally use through a range of motion are not used and therefore are not strengthened. This leads to strength imbalances which can, in turn, lead to injuries.
In extreme cases, the primary movers can become so strong in relation to the stabilizing muscles that they contract with enough force to literally tear those stabilizing muscles. To return to the Bench Press example, the pectoral muscles could become much, much stronger than the stabilizing muscles in the shoulder, leading to recurring shoulder injuries. Not a good thing.
The Smith machine will artificially inflate your numbers. If you regularly perform your Bench Press workout with 155 pounds with free weights, you can easily add another 30-50 pounds on in a Smith Machine. Conversely, if you bench 155 in a smith machine, you'll have problems lifting 125 with free weights.
Strength built with free weights translates readily to movements done in the Smith Machine. Strength built on a Smith Machine does NOT translate to free weights, or real-world strength.
Uses for the Smith Machine
Despite what I've said above, the Smith Machine is not completely useless. It makes an excellent adjustable chin-up bar (scroll down to Step 2) or inverted row station, and the loons over at T-nation thought up a few more uses, as well.
A good rule of thumb for using the Smith Machine: if the exercise involves you moving and the bar staying still (chin-ups, inverted rows, etc.), then you can use the Smith Machine. If the exercise involves the bar moving at all (Squat, Press, Bench Press, Deadlift, etc.), then STAY THE HELL AWAY FROM THE SMITH MACHINE.