Squat and hip extension

[Baldr]

At the start of the squat (when the knees are still locked out), should you been in any sort of hip extension? I don't think that the bar lines up over the middle of the foot unless I am in hip extension at the start of the squat, leaning forward, instead of keeping my back vertical.

[Mark Rippetoe]

I think you mean hip flexion, actually. No, you just lean forward a tiny bit to keep the bar in balance over the mid-foot, with the knees and hips in extension.