You should have a look into Arthur jones methods, he knew a lot more about HIT than Mentzer (seeing as though it was Jones training method).
I changed to hacksquats, and didn’t have any lactic acid lockups
I find the leg ext is the instigator, coupled with how the legpress pushes down quite hard on that particular muscle, upon lockout.
After I failed on hacks, I put on 25% more weight, then did a static hold at 3/4 ROM, until i couldn’t hold it and it turned into a slow negative. Took 21seconds for me to totally fail, and it was a very brutal and long 21 seconds.
Im really feeling it today.
PS: Been taking endura-max for a few days now. Should see some results next leg workout.