All right, all you biomechanists-to-be. Here's your first test. Can you tell the difference between Picture One (on left) and Picture Two? (Don't cheat...really take a look :).)
Take a close look at my low back and on the left you will see a lumbar curve (a "sway" in my back) and on the right, nothing but a straight line. Now let me tell you what's going in the muscles beneath the skin.
In the picture on the left, I've sent a signal to my transverse abdominals (TVA), which are muscles that, when contracting (shortening), lift the abdominal wall up toward my spine (or if standing, back toward the spine). Awesome. Now I have increased space in between my lumbar vertebrae, my glutes are in a position to be active (especially if I was upright), and I've flattened my abdominal wall, not only to look fantastic by the pool, but also to start a series of microbiomechanical changes that actually have a positive effect on plaque accumulation in the abdominal aorta (but that's for another time!) TVA activation is a win-win situation for all systems and your body really needs this muscle firing constantly for optimal function.
The picture on the right, however, is what most people do when they think they are using their stabilizing abdominal muscles. The beauty of biomechanics is how clear it is to see what's going on. If you see zero lumbar curve, then you know, for sure, the TVA was not used (there is no physical way it can tilt the pelvis). In picture 2, I see (and hopefully you do to) a pelvis that has moved into what is called a posterior tilt (tuck). The evidence is in, and the abs are out.
Most people have replaced deep, abdominal activity with "sucking their stomach in", and think they are constantly using their muscles. In actuality, the sucking in motion is a pressure (like creating a vacuum) that pulls the abdomen's contents up (not in), displacing the guts up against the diaphragm (hiatal hernia, anyone??). You get a flat stomach that looks fantastic by the pool, but you also get a tucked pelvis (do we need to talk more about where pelvic floor disorder comes from?), no butt muscles used when walking, no real work done in the abdomen, and excessive friction in the lumbar spine, hips and knees. That friction is called Osteoarthritis.
Here's another side effect of sucking it in: The constant, upward generation of movement caused by S.I.I. makes downward-moving processes like digestion, lower leg (and pelvic organ) circulation, and monthly menses more difficult for the body. For example, many people strain on the toilet because they don't realize they are keeping these processes at a stand-still. Prolapsing ladies, Stop Sucking It Up, and learn how to muscle it in instead. There's nothing that will push an organ out more than straining to bathroom. Check in with your abdomen and make sure it's relaaaaxxxxeeeed.
You're probably not going to enjoy the reality of letting your belly go. If you've got an extra 20+ pounds on your body that you've been sucking up, letting all of that mass out of your trunk (do you hear your organs breathing a sigh of relief?), will show you what you've go to work with. And, it's going to be ok. Here's the cool thing. That mass that you've been sucking in will go away now that you're allowing the muscles underneath it to work, increasing your metabolism with trunk, butt, and hamstring muscles. It will get better and you will be healthier, right away!
Now that your no longer sucking in, it's time to practice actual, TVA activation.
The Exercise: (Use a mirror if you've got one handy...)
Start on your hands and knees, with your hands and knees below your shoulders and hips. Let your head relax completely, breathing in a relaxed manner. Now, let the wall of your belly completely drop toward the floor (you get extra points if it touches!) and let your spine unfurl, making sure that you release your pelvis completely, lifting your sitting bones up toward the ceiling. Hang out there for 30 minutes. No, just kidding. But hang out for a little bit playing between where you feel most comfortable (your habitual position - watch what you do with your pelvis) and totally relaxed.
Now, exhale and pull your belly button up toward you spine, but do not let your pelvis move. See how long you can keep your belly button up, without holding your breath, without moving your pelvis, and without sucking it in. Can you feel the deep, abdominal contraction? Do this a few times every day in this exaggerated position, and then, take it to the real world. When walking and standing, do the same thing. When squatting with your tailbone out, you can also add TVA activation.
Moms-to-be, this is a great exercise to practice while pregnant as it can prevent (or repair) diastasis recti. Another fun birthing fact: The TVA is the best Birthing Muscle as the TVAs fibers works in the same direction as the Uterus does during the expulsion phase of delivery, only with a lot more force. So many moms-to-be are never taught about relaxing their abdomen and have deeply ingrained Sucking It In habits that are there when trying to push a baby out. Practice releasing now and make delivery that much easier! (That's my baby shower gift to you all :). I can't afford a thousand pairs of booties and onesies and other things that end in "-ies".)