Research on split squats and lunges shows that an upright torso position – where the shoulders are above the hip – is more quad-dominant than a more forward leaning torso position – where the shoulders are more above the leading foot.
This is relevant because a Smith machine squat involves the same basic torso/knee/foot position as a split-squat or lunge with an upright torso. It's simply a bilateral version of the basic movement.
Many trainers and coaches don't like Smith machine squats. They say they're not a great way to improve your squat performance since the two movements involve different mechanics. This is what we'd call a "straw man" argument.
Their reasoning doesn't make Smith-machine squats a bad exercise; it simply means they may not be great for the purpose of improving your squat without the support offered by the Smith machine.
But if you're trying to build and strengthen your quads, then the Smith machine squat is a great option.
- Farrokhi S et al. Trunk position influences the kinematics, kinetics, and muscle activity of the lead lower extremity during the forward lunge exercise. J Orthop Sports Phys Ther. 2008 Jul;38(7):403-9. PubMed.
- Schütz P et al. Joint angles of the ankle, knee, and hip and loading conditions during split squats. J Appl Biomech. 2014 Jun;30(3):373-80.