Popular for having a "built in spotter" with a catching mechanism, the Smith Machine may appear to be a safer and easier way to perform your favorite barbell movements but contrary to the popularity of this machine there are a few downfalls.
And, before you Smith machine lovers get your pants in a bunch, keep in mind this is my preference as a personal trainer (for the reasons I mention below), there are a few exercise I do enjoy using the machine for, and understand that everyone starts somewhere and this may help them to ease into barbell movements (see suggestions below).
The primary downfall of the Smith machine is that it's commonly used in place of barbell exercises but it doesn't actually move like a barbell.
The Smith machine is a machine. Although many use it as a free weight barbell, it is still a machine. As I mentioned earlier, due to the fixed plane of motion, the 15 pound bar on the Smith machine can only travel in a vertical movement. Mostly likely, in order to perform a squat on a Smith machine you're going to have to adjust your foot placement, which in turn will add additional (and unnecessary) stress on your knees and potentially cause your back to round at/near the bottom of the squat.
In addition, the fixed plane it also inhibits the body's ability to stabilize the weight as it's done the majority of the work for you. Specifically speaking about the squat, studies like this one have shown that muscle activation was 43% higher in a barbell squat compared to a Smith machine squat. In other words, a barbell squat naturally forces you to work harder to perform a similar movement. Similar studies found the same to be true for bench press as well. Don't get me wrong, I'm not hating on the Smith machine. Statistics like these are a dime a dozen in favor of free weights vs. all machines, not just the Smith machine.
Another downfall is that movement patterns of foundational movements developed on a Smith machine don't always transfer well to similar exercises down with a barbell. It's like trying to compare apples to oranges. A Smith machine squat/press/deadlift and a barbell squat/press/deadlift are not that same thing.
If I squatted with a barbell the same way I approach squatting on Smith machine I'd probably fall on my arse. In fact, just to be certain, I tried it. I found it nearly impossible to squat in my typically barbell back squat position without leaning into the barbell for support. Many will move their feet further out in front and lean back into the barbell on the Smith machine. This can be really great at placing a lot of focus and tension on the quads but also a lot of stress on the knees. I'm not saying it can't be done, but if you're squatting on a Smith machine, know that adjustments will have to be made if/when transitioning to a barbell squat.
Mark Rippetoe, a well known strength coach with decades of experience wrote in his book, Starting Strength,
"…barbells require the individual to make these adjustments, and any other ones that might be necessary to retain control over the movement of the weight. This aspect of exercise cannot be overstated- the control of the bar, and the balance and coronation demanded of the trainee, are unique to barbell exercise and completely absent in machine-based trained. Since every aspect of the movement of the load is controlled by the trainee, every aspect of that movements being trained."
On that note, if you're utilizing the Smith machine in hopes to gain confidence and move towards barbell movements, I'd recommend you skip the Smith machine altogether and start with body weight and/or dumbbell/kettlebell exercises instead.
Finally the comfort being the ability to rack the bar at any point in the lift on the Smith machine is a bit of a farce. I've seen the bar plummet to the ground before, it's not pretty. You can get hurt on a Smith machine, just as you can with any piece of equipment. If you're worried about lifting safety, it's a better idea to ask for an actual spotter as opposed to relying on the machine to bail you out. In addition, most squat racks have safety racks or pins you can adjust to protect you if you should fail on a rep.
With all of that said, there is no doubt that one could utilize the Smith machine to isolate muscle groups. For example, I've seen many creative ways of using this machine to target the glutes and quads. There are many options for accessory type exercises with the Smith machine. I personally find them great for rack chins (see first video below), incline push-ups and inverted rows.
And of course, I realize that sometimes, women especially, just need to feel comfortable in the weight room before they have the confidence to progress on to something else. If the Smith machine can help you to take that step, rock on.
If you're currently using the Smith machine for various exercises and are happy with the results, by all means, continue. But because you know I'm a barbell girl to the core, I'd encourage you to step away from the Smith machine and at least give barbells a try. Start slowly with body weight exercises, progress to kettlebells/dumbbells and when you feel ready, grab a spotter and try a few barbell movements.
Your turn: Do you prefer the Smith machine or a barbell?