Joint impact of ATG Squats

Question

Various weightlifting and exercise resources either claim that ATG squats (a**-to-ground, i.e. as low as you can) are perfectly fine or claim they will destroy your knees in two sessions.

I was wondering if anyone has any insight as to what is going on here. Will a properly performed (weighted) squat that brings legs below parallel to the ground have any greater negative impact on the knees or other joints compared to parallel squats?

Lets assume for a moment we aren't talking about competitive weightlifting weights, so with a squat weight smaller than 1.5x your body mass.

Personally I side with the "it won't do anything wrong" side, considering that

1. With bar weight at approximately your body mass you could exchange the bar with squatting on one leg, i.e. this is strain your body is "designed" to be able to withstand.
2. If you assume that there is the possibility of excess joint strain, you would be seeing epidemic levels of knee injuries among competitive weightlifters, who routinely go below parallel in the clean&jerk, with much greater weight, and while weightlifters can get various joint injuries, I haven't seen anything demonstrating that injuries are more biased towards the knee.

Answer 1

The majority of studies show that squatting just below parallel is perfectly safe and in fact, even beneficial to the knee. However, there is a difference between just breaking parallel and squatting until your hamstrings hit your calves (ATG). My personal opinion is that even ATG squatting is safe for the knees and studies of weightlifters - who squat as deep as humanly possible - generally confirm this.

However, there are some studies that show increased risk of knee injury from very deep squatting. Even more importantly, the benefits from going ATG vs. just below parallel aren't huge for most goals/activities and not many people have the flexibility to do so while maintaining a proper lumbar arch. Therefore, while I'd always recommend squatting below parallel, I'd only suggest ATG squatting if your goals directly benefit from it, with olympic weight lifting being the primary use case.

For your reading pleasure:

• exrx - Squat Analysis
• Deconstructing The Squat Part I
• Are Deep Squats a Safe and Viable Exercise?
• Injury rates and profiles of elite competitive weightlifters.
• Knee biomechanics of the dynamic squat exercise

Answer 2

All I can recommend is to try it out and see, I often do squats like this. I never called them ATG Squats though, its the first time I have seen the term ATG Squat to me it was just a very deep squat.

Will it cause injury to your knees I am not sure, performing squats is all about technique. If you get the technique wrong then the chances of injury are high.

As mentioned by @md5sum How deep should a squat be?.