abdominal muscle engagement mechanics

I have been sitting here this morning on my computer stool pondering engagement of abdominal muscles.

When I lift my chest, I lift it from my lumbar spine first, going upwards. This makes my chest first go forwards, then up, then back. when I move my upper spine back, my shoulders stay forwards.

If I do the reverse, and tuck my chin and move my upper spine first, I can feel pressure in my vulva, and my belly protrudes first. My chest does a vertical circle but in the opposite direction, going down, back, up and forward. The latter method is very convoluted and does not feel at all intuitive to me. I will stick to the bottom up approach.

The other thing I notice is the way my different abdominal muscles literally switch on as my chest raises further and further. I think it is obliques that become quite hard first, then transverse abdominals deeper inside, then rectus abdominus last of all, which automatically springs into action at the last minute (boing!) and stops my chest rising any further. If you stand or sit with your thumbs at the sides of your waist and your hands at the front of your waist so your middle fingers are trying to meet up, then raise your chest, feel carefully for sensations under your hands and feel each muscle group engaging, one by one.

Then repeat the exercise, this time just concentrating on what is happening in your pelvic floor. The first thing I feel is a pulling sensation in my vulva, possibly the labia at the same time as my obliques engage. This may be the round ligaments pulling my uterus forwards, as Christine explained in another topic the other day. Then, at the end, when my rectus abdominus kicks in, my PF tightens longitudinally. At the end I can feel my tailbone being pulled forwards as my PF tightens.

(Is this right? I don't think the levator ani is joined to the tailbone, instead going around the back of the anus and forwards again, so that the anus gets pulled forwards. Why can I feel a pull on my tailbone? What muscles contract to allow the levator ani to relax and permit the rectum to move backwards and into line with the rest of the large intestine and to empty in a straight line? ie what is the LA's antagonist that pulls the rectum back for emptying ?)

The levator ani has to engage because (I think)it is the antagonist muscles of the rectus abdominus. When the RA contracts in response to chest raising, it tries to pull the pubis symphysis forwards. The LA says, "whoa back" and contracts to counteract the movement of the pubis symphysis. It is like a 'tug o war' mechanism. When one side relaxes, the other side relaxes. When one side contracts the other side contracts to keep the centre point of the rope (pubis symphysis) in its correct place.

If you tuck your butt, which counternutates the pelvis, it automatically loosens the pelvic floor (drumskin). Therefore the RA will automatically loosen as well because it no longer needs to remain engaged to keep the pubis symphysis back in its spot.

If you tuck your butt all the time your RA muscles will never be engaged during everyday movement, so they will lose tone and your belly will protrude. I think this is why fitness world is obsessed with exercises to flatten the belly!

Christine and other anatomy-curious Members, comments please.

Louise