I am overweight. Morbidly obese would be the current term. I still harbor the illusion that I just need to shed a few pounds. But there was a time I was very skinny. I never had a 6 pack but I could whip out a few hundred "situps". I think instead of using the abdominals I was using the hip flexors in more of a closed chain action. All muscles can work in two directions depending on which side is fixed. In the old days a person would squat by your feet and grab you ankles pressing them down making your legs fixed and the trunk free.
(drawing shows one side only there are two)
I bring the psoas up because I don't think I had a clear delineation between my left and right side in my use of my body. Playing with the left tongue has given me more of a sense that I have two groups of muscles close to the center line on my anterior and posterior surfaces(Stomach and spine).
I am not sure why. I imagine there are many people who have had this sense all their lives. It was not just the exercise that I needed to get a different sense of myself because that just repeated what I had already habituated in my movement patterns. In fact effort was probably counterproductive in some ways. Even now if I tried I would switch rapidly back into my older more familiar sense in any sustained effort. It would probably be much more healthy for me to be exercising but there was always a sense of doing something not correct.
Hope springs eternal
maybe If I get off this couch...