Postulates: 1. DL jumpers need strong quads. 2. Strong quads help us jump and run faster in athletic activity. 3. Athletic squatting helps build stronger quads faciliating 1 and 2.
The above are taken for granted on first principles.
Argument: When squatting a challenging rep near RM, squats turn into good mornings. This suggest hamstrings are not able to maintain back angle. This means hamstrings are not strong enough. In the past on this forum this was rejected and it was proposed rather it was quads which were not up to the task. I was told to front squat because this would build up the missing quad strength and bring enough leg strength to the table. The prescription/antidote was correct but the explanation was wrong (explained below).
New information: Whenever i've used a belt, i find my form deteroriates instantly, even in first reps not close to RM. I never understood why, however, with the study that came out recently (linked on this sub forum), it was suggested a belt helps us squat better by allowing the legs to work harder than sans belt. That was the missing link in the explanation for my back squats with a belt -- by using a belt, my hamstring were overloaded and were not able to maintain back angle! Hence squat mornings when using a belt.
Fix: Make hamstrings bigger and stronger.
Role of front squats revisited: It turns out a rep limit set also exhibits the problem described above, namely to maintain back angle. It just so happens front squats require a upright back but nevertheless the hamstrings are responsible for maintaining this back angle. By doing front squats, i wasn't 'bringing up my quads' as suggested but rather, i was training my hamstrings to be stronger and better at maintaining back angle!!
Future exploration: How do i do an even better job of bringing up my hamstrings so i can use a belt and become a big squatter (180kg)? That's where i'm stuck .. i know front squats help but it feels somehow not enough. There might be other ways to build up hamstring strength to maintain back angle. An idea I had was simply to do more belted squatting and force my hamstrings to be exposed to more loading and hence learn to maintain back angle in that overloaded situation. Interestingly, unlike many others who use a belt to lift more weight, i would have to use weight less than my beltless squat, since that would address my weakness better.