I've often wondered what creates that bent over posture as a person ages, then noticed that many middle aged folks already are leaning in that direction (no pun intended), and even younger people can develop a flexor habit that will begin to shorten the anterior of the body. Flexor habits (when the muscles that flex the body - like the abdominals - can begin with a growth spurt early in life that makes the adolescent so self-conscious of their height that they actually try to make themselves look smaller. Girls may begin to curl under to hide the development of breasts at puberty. For some more conservative outlooks, it's not acceptable or appropriate to show a butt that sticks out, and many women used to wear a girdle to squeeze it back in.
When working toward a six-pack, the abs and psoas naturally are engaged and become tightened and tone to achieve that look. The anterior fascial line will also become engaged further shortening the front of the body. It's a pretty swag look initially, and most jocks have it, particularly if they lift weights. Over time, however, unless there's a balance in the workout the rib cage will drop, the shoulders will roll forward and apply more weight to the anterior frame, causing the pelvis to roll under. Working in a seated position for a long time can do the same thing, shortening the same muscles (without the benefit of the six-pack!).
When I noticed my body beginning to 'drop and roll', I thought, "Uh-oh - this can't be good! What's causing this and how can I unravel it?" I realized that the skeleton was looking for stability, and having my thigh bones wobble in the socket didn't feel stable over long periods, so my pelvis curled under so it could lock into position against the back of my thighs, and when the pelvis curled under, my chest had to drop to counter-balance. If the chest drops, the head and neck push forward and quite some strain happens in the neck, efforting to keep the head lifted without the skeleton underneath. Over time, kyphosis would develop at the back of the base of the neck, adding bulk to carry the additional strain of the head being forward. We've all seen this posture, even on young people, but if it sets in and changes the shape of the spine as senile posture, it's much more difficult to change. Nip it in the bud!
After trying many solutions, the easiest one arrived for me: if I lifted my chest, my pelvis naturally arched my back to balance, or I could arch my back and my chest would automatically lift. When the chest landed in a stable alignment over my pelvis, my legs felt solid and didn't wobble.
Sometimes people think I'm adopting a rigid 'military' posture, but it really happens naturally when I reposition either the thorax or the pelvis, and no muscular effort is needed. It was the same solution to more effortless jogging. I still have to avoid locking my knees in that search for a stable stance. A little 'rocking' is natural and gives good input to the brain as it continually sends balancing info. This alignment also provides much better energy flow up the core, more circulation through the neck and better alertness throughout the day.
Doing a few cobras and camels to open up the area between the diaphragm and the pelvic floor will help the effortlessness of the shift, but if those internal long ligaments, sheaths and deep fascial planes have shortened, you may need a little professional bodywork to open it up first, or some guidance by a trained yoga professional.