The Rat and the Mongoose, the Ribcage and Pelvis
The mongoose was introduced in Hawaii at the end of the 1800's under the pretense it would manage invasive rat populations on sugar plantations. The mongoose is quite a predator, and at the time farmers in the Carribean felt successful with the approach. But the rat and the mongoose are on different schedules, one working the night shift and the other the day. So while a mongoose may not mind a rat snack now and again, birds and their eggs are the real feast, including the endangered native birds. Now, Hawaii has a rat problem and it has a mongoose problem and another threat to native species.
Strange as it sounds, I sometimes see an unhealthy and dysfunctioning pelvis/ribcage relationship in a similar way... which end is the original problem depends on the population. Often the tucked pelvis is the rat, and the swung forward ribcage plays the mongoose. But sometimes the swung forward ribcage inspires the introduction of the tucked mongoosy pelvis.
Why the posterior pelvis? Decades of tail tucking exercise classes - from sweaty gyms to glitzy studios. Lots of sitting. Lady training. Dance training. Man chairs. Driving. Shoes. Runways. Shame. Withdrawal Reflex. Psoas tension. Weak legs. Stress. Tight hips. Maybe just a deep tension at the very core of the body. Not necessarily the muscular core so marketed these days, but the midline, the nervous system core.
Why the swung up, forward thrusted ribcage. The desire to feel uplifted. The desire to look uplifted. Yoga ideals. Beauty. Performance. Power. Psoas tension. Work. A deep tension at the midline of the body, at the diaphragm, in the nervous system.
Which arises first may be individual, but the result is the same. It's a pattern Thomas Hanna called the 'dark vice', stuck in green light reflex (action) and stuck in red light reflex (withdrawal). No matter what you call it, it has the potential to be pain-making at the SI joints, the hips and knees, the low back, even the neck. And it's certainly freedom limiting.
In yoga practice, this pattern shows up for a few common reasons. Modern hips sit a lot and are often tight, so the tendency to roll back toward the tailbone, passively posteriorly tucking the pelvis, is strong (even in hyper mobile folks), especially while seated on the ground. Add to that mix : cues to lift the chest or tuck the tailbone, or just the desire to feel more uplifted, and you may now have a lot of unnecessary tension in the deep channels along the spine.
And our nervous systems are sometimes just wired, exhausted, busted. Bringing the center channel of the body and the relationship of pelvis and ribcage back into harmony requires something other than stretching and other than 'core' exercises. Many somatic methods are helpful. But if that's not your jam you could also try the following asana..
Shant Sarovasana,the pose of a peaceful lake. Lying down on the back, with knees bent and the feet about pelvis width distance apart, sense the weight of the pelvis, sacrum resting on the ground. Sense the weight of the back bottom ribs (TLJ) releasing towards ground. Sense the shoulders, skull, arms and feet resting into the ground. You may find the tail tucking, the back waist flattening and/or the bottom ribs lifted up off the floor for a while, if that's what your system knows best. Stay and breathe gently, letting both ends to gradually release into gravity as you allow the contents of the abdomen to soften and descend. Eventually, the lumbar curve is restored but the ribcage releases to gravity. Sense the ease, peace and tranquility available in the depth of the lake of the abdomen. If the legs fall apart, belt the thighs at pelvis width. If they fall together, place a light block, ball or rolled towel between the knees. Sometimes the legs shimmy and shake in this foreign territory. The props will help let go during initial stages. Stay for a few minutes if possible, gradually increasing duration with practice. This un-work will serve you, especially if you are super active in the world.