Some of the more important muscles of our core that provide this stability are our transversus abdominus muscle, our lower back muscles, our oblique muscles, our quadratus lumborum muscles, our pelvic floor muscles, and our diaphragm. (hab-it.com)
When the squat gets heavy and your hips shift to one side, the quadratus lumborum muscle could be tight on the side it's shifting to. (getthisstrength.com)
And with all that engagement through the hip flexors and the Quadratus lumborum of this position when a true back without all that rounding, we're going to add to it with the rectus femoris (that's the hip flexor that's in the quadricep muscle group in your thighs). (weckmethod.com)