Personal trainers who have clients squat in the Smith machine are lazy, ignorant or an unflattering combination of both.
The squat is the king of exercises and I have written about the importance of working with a personal trainer who knows how to teach this exercise.
There is a reliable progression trainers must follow and things trainers should not do when teaching the squat. If your trainer has you squatting in a Smith machine they are doing you a disservice and you should ask their reasons for putting you in this unfortunate situation.
Part of the excuse/explanation will likely deal with helping balance and stability while getting to the proper depth with proper form. Which are all wrong answers/justifications.
Every day we move around in three-dimensional space so – as much as possible – we should exercise in this environment, especially when performing a ground-based, compound movement (GBCM) like the squat. To squat in a machine would be like throwing a baseball while locked into a machine. It’s ridiculous to think of someone throwing a baseball or football, swinging a bat or a golf club or shooting a basketball in a manner dictated by a machine and yet many people exercise this way.
When you squat using the Smith machine, the machine is providing you with balance and stability and as a result you will never improve in these vital areas. As far as getting to the proper depth with proper form, a good personal trainer knows how to get clients to do this without the aid of a machine.
A trainer certified by a legitimate organization knows how to get you to squat. As a matter of fact, thanks to the Internet anyone who is interested can learn how to squat properly. Performing the squat – and other GBCMs – in a machine puts stresses on the musculoskeletal system that are potentially injurious, because the body moves in a manner that’s determined by the machine.
Squatting in the Smith machine forces the body to move in an inflexible, linear fashion and not the way the body should move while performing this exercise. Joints and connective tissue are subjected to forces squatting in the Smith machine that are not encountered when doing squats properly. The squat is an exercise that needs to be performed properly. If your personal trainer does not know how to teach you squats, and does not squat properly in their own workouts, find a new trainer.