Now in this drill I can flatten my lower back on the ground, but as I squeeze my glutes my lower back comes of the floor a little bit. I'm still engaged in PPT but I just can't press it on the floor. It's anatomally impossible for me.
Is it thigh hip flexor? What are the boundaries here, really?
Can a woman with a big ass like this girl:

ever flatten her lower back on the floor with squeezed glutes and straight legs?