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I think it’s mostly ankles, because almost everybody has a better squat with the heel elevated!
But the akles are very hard to improve if you are restricted.
If you have a tendency to fall backwards or round your back it’s probably ankles and/or hip flexion.
But if you want to really squat like kelly you also need good hip rotation in flexion and abduction, smashing these muscles will also help with that. You can also do the Olympic Wall Squat kelly uses a lot.
But than you also need to look at the hip capsule. IR in flexion is restricted by many people so doing banded distractions pulling the femur back or to the side with the leg in internal rotation will help!