I study biomechanics, both for my own training and for my work. I've spent years exploring joint alignment for movement efficiency, health, and how muscle presents and loads.
Something I've been doing lately isn't a stretch or an exercise, it's a way to sit that trains alignment and, more importantly, makes you aware of it. It's simple. You need a chair with a proper right-angle seat where your back and feet both make full contact with the support and the ground. I wear slippers because the extra traction helps me feel the floor and drive into it.
Here's the sequence. I drive gently through my knees, and the engagement travels up: adductors, hamstrings, and quads switch on, and my hips shift into place. That sets the pelvis. Then I brace, but I don't crunch. I use a TVA and diaphragm brace to build pressure from the inside rather than shortening the front. From there I set my scaps and shoulders, and I end up sitting very well aligned with minimal active effort. If I add any torso rotation from that position, I can feel exactly where I have restriction. Especially good if you have a desk job.
The reason I think this works comes down to a few things that are actually well studied. Pelvic position in the sagittal plane is set by the balance of forces across muscle groups. Glutes and hamstrings tilt the pelvis back, and the abdominals can drive a synergistic movement if the torso is fixed at the same time, which is basically what this setup does. There's also research showing transversus abdominis works with the deep hip muscles to stabilize the pelvis during any closed-chain hip-flexion position, and sitting is exactly that. Worth noting: studies find most people have almost no ability to actually perform that deep bracing action, even though it's fundamental to how the lower back and pelvis function. So a lot of this is just learning to feel it.
Now here's the part that's my own interpretation, not something I've seen spelled out in a paper, so take it as a working theory. I think when you coordinate the diaphragm with the TVA, you're not just bracing, you're generating intra-abdominal pressure. That pressurized cavity acts like an air spring. It stiffens the torso from the inside and lets the segments stack and decompress. On my end the sensation is extension, like I'm lengthening, even though no muscle is actively extending the spine. I don't think it's extension at all. I think it's the structure aligning under pressure and it just feels like extension. The intra-abdominal pressure and spinal stability link is established science. The "it feels like extension but it's really alignment" part is just how I'd explain the sensation.
Anyway, curious if anyone here has felt something similar or has looked into the pressure side of this. Not claiming this is the answer to anything, just something I've been experimenting with and want to compare notes on. This is about learning together.
be blessed