I am 44, 250 pounds, been lifting for many years. Not a regular squatter, but I have done them on and off. I've tried deadlifting before too and have the same problem, related to a "bad back" or "weak" back.
I hurt my back in high school football ("wrenched it"). Hurt it again in college (poor squat form)...took an army physical in 1990 and FAILED it, due to "bad back"...they didn't say exactly what was wrong, they watch you walking with your hands grabbing your ankles and said something was wrong.
Anyway, I try to do very "good" squats, leaning back, keeping feet at shoulder width only, keeping shins perpendictular to the ground, going "all the way down", no bouncing at all, in fact I try to pause in the hole...but the problem is I can't do much weight. Like 135 for 12 or 15 about wears me out. More and it a) feels heavy when I rack it...like unstable.
When I use 135 I can feel it in my hamstrings and hips but more than that it feels like my hamstrings aren't firing or something...I end up leaning forward on the way up (not sure if that's good or bad)...just don't feel as "in control".
I remember in college the doctor told me that I had a "bad back" that the bottom vetebrae were made of cartilege instead of bone (not sure exactly but that was the gist of it)...later another doctor (an ortho) said he'd never heard of that.
Any ideas what is wrong with me? I've been to doctor's before...one had a reverse hyper type of machine which I couldn't do at all...they didn't diagnose anything anatomically wrong with me, but I can't figure out why my back is so weak if I am doing things to stregthen it and the rest of my body is relatively strong.