«When you're pelvis this region starts to sag down while doing this exercise» should be «When you're pelvic region starts to sag down while doing this exercise» (ashotofadrenaline.net)
When you're pelvis this region starts to sag down while doing this exercise immediately tighten your abs and get back into a straight line. (ashotofadrenaline.net)
Especially the pains in the back and pelvis region. (pillowsforlife.com)