lumbar flexion in the back squat

Hi,

I cannot maintain a normal lumbar curve when descending to the bottom of a back squat. I always exhibit a tiny bit of flexion at the bottom, despite cutting depth to the absolute minimum, and on video my back clearly changes shape starting a few inches above parallel.

According to Starting Strength, 3rd edition, this is improper form. However, I have not been able to fix it. I use lifting shoes (with a raised heel) and a belt, and I have tried various combinations of stance width and foot angle. I have voluntary control over my lumbar erectors and have no problem setting my low back on the deadlift. I have a short torso and long legs (relatively).

I have no pain or low back issues, but I do feel I’m losing tightness at the bottom and would be stronger if I could resolve this problem.

Is there any other intervention or cue I could try to improve my lumbar position? Or should I just accept that there will always be a bit of flexion due to my anatomy and soldier on?

Thanks as always for sharing your expertise!

Best,
Patrick

(I posted a similar question earlier, but it was fairly garbled, so hopefully this is more precise.)

Video?

Have you been coached?