Bench presses are such a basic exercise you might not think that there is more than one way to perform them, or that you have a choice in equipment. But you do, and one example can be found in many gyms: a piece of equipment called a Smith machine. Because of the way it's built, there are advantages and disadvantages to using a Smith machine to bench press instead of free weights.
Types of Weight
If you're not limiting yourself to the Smith machine, you also won't be limiting the choices you have in what type of weight you want to use. When you bench press using free weights, you can either use dumbbells or a barbell loaded with weight plates. The Smith machine features a barbell affixed to a sort of power rack, so the only choice is how much weight you want to load. This is where the Smith machine is similar to a barbell for a bench press. You just place the weight plates onto the machine's barbell, attach a collar on each side for safety and start pressing.
You should always have a spotter when you're bench pressing with free weights. If you're spotter is out of town on chest day, though, you'll have a built-in spotter if you bench press using a Smith machine. The barbell is attached to a track, so if you are stronger on one side than the other, you won't have to worry about the weight tipping the bar to your weaker side. Also, the Smith machine has a series of holes drilled into the frame, along with rotating hooks attached to the barbell. If you get into trouble with lifting heavier weight than you can handle, just a twist of the barbell will lodge the hooks into the closest set of holes, securing the weight and allowing you to slip out from under it safely.
If you alternate between a Smith machine with free weights, you'll notice that you'll be able to lift heavier weight with the machine. That's because the track that the barbell is attached to lends a degree of support, so you don't truly have the full weight of the plates resting on your body. With free weights, however, once you un-rack the barbell, you have to support the full weight.
Movement and Muscle Activation
Your range of motion and the muscles that are activated to work are affected by whether you use free weights or a Smith machine. The track on a Smith machine gives you more control over the weight, but it also limits your range of motion, keeping you from pressing your arms up in a natural way. This also makes it unnecessary for your stabilizer muscles, such as your scapula and rotator cuffs, to activate to help support and press the weight. A study done by the California State University Department of Kinesiology and published by the Journal of Strength and Conditioning Research found that the anterior and medial deltoids and the pectoralis major were more greatly engaged when performing a bench press with free weights than when using a Smith machine. The researchers concluded that free weights are more useful than the Smith machine for building muscles and strength in the upper body.