This is a subject that is close to my heart, and unfortunately very bad for my back. Before we get into the main discussion, here is a little background on why I don't squat anymore. About 5 years ago, I had a bulging disc in my back, which made my foot drop (go floppy), and made my life very difficult for a long time. This happened for a number of reasons, a bad bed, a poor strength coach, and a young rugby players belief that he is indestructable..... but not from squatting.
The reason I can't back squat any more is that this injury has caused my lower back to be in extension all the time (arched), so when I perform the squatting movement my back does not let me achieve the right position.
Because of this I have spent a lot of time with various physio's and strength coaches discussing the benefits of back squatting, how deep they think you should squat, and other exercises that could, if need be, replace the back squat. The results in general have been hugely divided, most strength coaches love the exercise, and believe you should squat deep to get strength through your full range, for full glue activation and ultimate gains. There are a few strength coaches however, and most of the physio's/surgeons who believe that loading the knees through that range is a knee injury waiting to happen. Most of these guys, especially in rugby, think that you should only squat to 90 degrees, because this is the range you generally go through in a game.
My view on this has changed a few times, but after listening to a number of opinions, and watching my clients and athlete's perform the exercise, here is my view..
IF you can squat deep, ass to the grass, with your back dead straight, your head and chest up, sitting into your heels,and keeping your knees facing outwards on the way up, go for it. I believe that in this position the stress is taken from the lower back and the knees and huge strength gains can be made.
HOWEVER if when you squat
-you lean forward putting pressure through your lower back
-you crumble through your trunk
-you cannot keep your chest high
-Your knees fold in on the way up.. get some professional help to gain the mobility and strength to perform the exercise well.
Some people will never get to the point that they can squat perfectly, me being one, but if you can do it well, it can be beneficial.
If you are doomed to be a non squatter, do not be disheartened, I have found exercises like front squats, leg press (controversial), single leg squats/lunges and deadlifts keep my legs happy, but if I could squat again, I would.
For more info on me and what I do visit www.razor-fitness.co.uk