A while back there were a few people posting about
low back pain. The internal psaos/iliacus are often ignored, even by the muscle "professionals". These are ARE THE CENTER of your CORE. Under and to the side of ab muscles, and stronger than abs. I would suggest goggling these in "image mode" to see these if you are unfamiliar with them.
A clue to these muscles being sticky and/or tight, is ache in the low back after a period of crouching down from gardening, auto repair ect. Sitting shortens these to and you feel it in the back when getting up.
Very few people are aware of these or how to stretch them. Regular hip flexor stretches dont do squat for these internal muscles that attach the from lower hip to the lower-mid spine. Its a tug of war and your spine will lose. The iliacus is in the "dished or cupped front side" of your hip bone. The Psoas lies next to close to center (it attaches to spine) it and is longer.
They can be accessed by laying gut down on soft ball or tennis ball, or on back or side with :thera cane tool, or your hand. Bringing your thigh closer to chest shortens this muscle. Arching your back with leg behind lengthens it. This wont do much by itself, but when incorporated with a deep hand or thera cane, the results can be spectacular (this is what I do).
these you tubes will get you started:
/www.youtube.com/watch?v=R9nHxDigGOI/www.youtube.com/watch?v=8s5kBCZgFbs/www.youtube.com/watch?v=gWLnAG-G6ts/www.youtube.com/watch?v=SSw10kVPf60