I need some advices to understand if my low back posture is correct while squatting.
Let's consider these pictures of me, referred to the way up of squat:
Picture 1): I've arrived at the parallel. I don't usually try to go too much lower, since it's difficult for me to keep the correct position during this phase. So, since I'm learning, I want firstly to improve my general posture during squat. In this picture, my lower back has its natural curve (natural lumbar lordosis), so I think it's fine.
Picture 2): I'm on the way up. Let's focus on the red circle. As you may see, for an istant there is a little bulge on my lower back. I've noticed that this happens not only in squat, but in general when my abs are squeezed and my back is not extended forward. If I try not to squeeze my abs, this bulge won't exist, but of course abs must be kept thight during all the exercise.
Picture 3): I'm arrived at the final position. The low back curve is natural, so I think it's fine.
As you have seen, my doubt regards picture 2. Why does squeezing my abs cause such a little bulge? Is it physiological? Is it correct? Is it natural? Or should I do something to avoid it.