I'm familiar with Bill Starr's rehab protocol. My problem isn't so much the rehab. I've been rehabbing and having many setbacks, so I've decided to slow play it.
My problem right now is that I can't keep from rounding my lower back at the bottom of the ATG squat. If I contract my hip flexors really hard and squat slowly, I can prevent the rounding, but I don't think I should have to squat that slowly to avoid rounding.
I'm thinking my problem may still be tight hamstrings. I do make sure that I can get to at least parallel in the GM stretch before I squat though. And, I SMR my piriformis.
Which is why I'm wondering just how flexible people who ATG squat are in their hamstrings.