I had really tight hamstrings. I have been stretching them for quite some time now and it improved remarkably.

When I look at this picture I notice the straight upper an lower back.

My problem is that my butt always tucks under when I squat (anterior pelvic tilt, "butt wink"). The same applies for the shown stretch. So, my idea was that both problems are somehow connected.

This is not a picture of me, but I look basically like this (except for being able to almost touch the ground with the palms of my hands):

But I can clearly feel that I compensate lacking flexibility with spine rounding.

So, is this still a hamstring problem or is it another muscle? Please give me specific stretches to achieve the position shown in the first picture.

I had really tight hamstrings. I have been stretching them for quite some time now and it improved remarkably.

When I look at this picture I notice the straight upper an lower back.

This is not a picture of me, but I look basically like this (except for being able to almost touch the ground with the palms of my hands).

Now you will say, do the earth touch more, but there is a problem with it.

I can clearly feel that I compensate lacking flexibility with spine rounding. I feel, that if I only do the earth touch only my spine gets rounder over time.

So, is this still a hamstring problem or is it another muscle? Please give me specific stretch to achieve the position shown in the first picture.

Edit: Removed air squat part, because the focus should be first on the given stretch otherwise this thread gets to complicated in my opinion. I have a concrete problem at hand and I want to solve only this first.