I'm not sure where the burden of proof lies in this case. It's hard to judge which of the two claims "heavy squats will cause harm if done for many years" and "heavy squats will not cause harm even if done for many years"
I'm not too sure either side is transparent enough to narrow both points down this far, yet.
Earlier, I attempted to highlight the difference between "damage" and "pain". Or atleast, emphasise that there IS a difference between the two. Unless I missed it, that point has either been overlooked or, brushed off with, "you will realise i'm right when you're older".
I can't really define my own point without knowing what "harm" is actually defined as.
Lets say pain/injury are what "harm" is and get back to the squats. If you squat heavy for years you will cause harm and therefore injury if you keep doing it long enough. First of all that doesn't hold water with me since we know, with some statistal evidence, that 80% of the general population will experience back pain at some point in their lives. So, if I squat heavy I will get back pain. If I don't squat heavy, I will also get back pain. The claim may be that there is MORE risk of pain if you squat heavy. In my mind, this is quite a significant assertion with no evidence.
Who would you test and how would it be quantified? I have actually agreed that most untrained people shouldn't do heavy back squats, plus loads of trained people. Not for the same reason, though. I don't think most have the movement integrity to pull off a well executed back squat, and this must be developed first, if it can be developed at all, and even then, it still has to be justified within the goals of the client (as does the leg press, which i've still not heard a good reason to justify doing, other than hypertrophy). I think this is also a point refuted by the prediction of future realisation - i'll learn this when i'm older, too.
Also, surely, if this was the case, you wouldn't get lifters with back pain who then get rid of that and either resume, continue, or start back squatting. If back squats were the mechanism, then should this even be possible?
Really, if harm is being defined as injury or pain, then my response is -"It depends, and there is no real reason to think that", I've not seen one in this thread and didn't see it in the video, either. It seems more like a guess, based on analogy.
Now lets say harm is defined as "damage" (wear and tear). So, doing heavy back squats will accelerate damage to the spine. This is an interesting and surely immeasurable claim. Now, all other things that can cause damage/wear and tear to the spine held constant i.e. you have intendical twins who lead the exact same life, same activities, jobs, etc. One heavy back squats but the other doesn't. Then, yes, absoloutley, the one who squats will inflict more damage. This is basic maths, not even biomechanics or physics. One twin's spine does more stressful things than the other twins spine.
My response here, if this is the case - So? To expand, I need to go back to the point concerning the difference between damage and pain. It seems, and with some evidence to support, that damage does NOT = pain. I linked an example, too. They're not the same thing.
My diplomatic response is, "i think we're missing the forest for the trees". I'm saying that for someone with a good base of movement, with the right goals, a good training program, squatting with propper technique, has no reason to fear that this will lead to a back injury.
It gets even more confusing, since I still don't have a definition for, "what is heavy"? Are we talking about training for a powerlifitng comp, or just general strength? I did ask this before, but I did type A LOT. There is significant less risk when you are not squatting for competition.
Movement, as far as I can tell, isn't even being considered, yet when discussing pain or injury, particularly in the context of mechanisms for pain, surely it should be of great significance. For the layman, in this case, think of someone who butchers a body weight squat yet decides to load up heavy on back squats. Well of course he's more at risk - he's practically begging for pain. In my limited experience training people so far, most untrained people fall into this category. If you make these people back squat, then you will inevitably come to the conclusion that there's less risk involved in leg pressing and I can see where you're coming from.
I guess my argument is that it's a very vague and unsupported claim to make.