Originally posted by: brikis98
I think the key issues are as follows:
1. An increased ROM on an exercise typically makes the exercise more effective by involving more muscles and more muscle fibers. If you could ignore the limitations of injuries & flexibility, then for general strength training the maximal ROM of the ATG squat would probably be preferable.
2. However, you obviously can't just ignore injuries & flexibility. To ensure safety during the squat, you must maintain a proper back arch and should keep all your muscles - especially the glutes, spinal erectors, hamstrings and adductors - very tight. It's worth noting that you can't properly engage the hamstrings and adductors, and consequently can't keep your knees safe, unless you go below parallel. It's also important to keep the knees/thighs directly over the feet as seen from above, to keep the heels flat on the floor, maintain a neutral head/neck position, and so on.
3. Putting the above two together, you should use the maximum depth in your squat that still allows you to maintain proper form. Most people can't even break parallel while maintaining proper form, let alone go significantly below parallel. In fact, I'd guess that 99% of trainees could not maintain proper form for an ATG squat - to get that deep, they'd end up relaxing their lower back, loosening their hamstrings, coming up on the toes, and so on. Since there is little question that sacrificing form and risking injury for depth is a bad idea, for most people attempting an ATG squat is NOT desirable.
4. That's why my recommendation is always to go as low as you can while maintaining proper form, as most people can build up the flexibility and technique to do a slightly below parallel squat, but not a whole lot more. For those few rare people that can actually go deeper with proper form, as long as they build up to it gradually, I suspect that the injury potential is quite low. However, I must admit that although I've seen research indicating that below parallel is clearly safer than above parallel, I haven't read any research comparing below parallel squats to ATG squats. Still, many olympic weightlifters perform ATG squats regularly and I'm not aware of them having an epidemic of knee problems - even though the catch & squat portion of a clean/snatch is probably much faster and more violent than a regular squat - so I doubt there's much to worry about. I suspect that even if going deeper in a squat increases the stresses on the knee, those connective tissues adapt to it during training in the same way muscles, bones, and the nervous system do.