Hi Iceman, I have seen a physio who specialised in pelvis biomechanics and he said my right side of the pelvis was rotated forward a bit and that was putting a strain on my SI joint and giving me pain in the ligament on the rhs of the SI joint.
He gave me a muscle energy technique stretch to help this where you lie on your back and bring your knee up to your chest.
I think it was just my SI joint he talked about when he examined my lower back.
I stopped seeing him when I started getting knee pain because he said it was to be expected as I was re-strengthening my muscles, but I was struggling to walk and it was getting worse. And it turns out I was developing tendonitis above my knee cap and I should have been resting.
Another physio I saw a few years ago was unsure about my lower back and used to push into it with her fingers and say it was tight and a bit higher up it was too flexible when she pushed in. But she wasn't sure why this was but did wonder if it was linked to the nerve pain I was experiencing in my calf. She gave me neural stretches which helped my calf improve.