Ok, I've come to the extremely unpleasant realization, that even after my back-off period, and trying to focus on the SPEED of my descent, I have never actually figured out how to tighten my trunk for the squat properly.
Reviewing some of my videos, I noticed that my knees and hips go back, making my back more horizontal as I start up almost every time. I found this error going back in the blue book, in the Hips section, under Common Problems, page 45, last paragraph, left column. The first thing in the anatomical discussion that follows is that the pelvis angle/low back set is not being maintained. I realized that I can't remember the last time I consciously set my lumbar! I just "tighten up", and go. So here is my second set, with me trying to focus on maintaining lumber extension:
Squat - 310 - Set 2 - YouTube . I felt some muscle burn in my glutes that felt new, but it looks to me like my knees and hips are still going back!
The next major topic covered is the necessity of external rotation of the femurs (knees out), to allow depth without having to flex the lumbar/tilt the pelvis. So here is me trying to think hip drive, and hold my knees out as I start up:
Squat 310 - Set 3 - FAIL - YouTube . You'll note I wimped out on the 4th rep.
I've got too many damned variables running around my head, and nothing I'm trying to focus on seems to be helping, but I'm such a rank novice and physical idiot, I could be missing something obvious. For pete's sake, y'all have been telling me to slow down and stay tight in my descent for 3 weeks, and I JUST figured out that I wasn't paying attention to lumbar extension!
Any help appreciated.