I have a questions about what abdominal muscles I actually have, and what they do.
On one hand, I often hear about the distinction between outer and inner abdominals: The outer abs are mostly worked by crunches, the inner ones by exercises like the plank or lying on the back and slowly lifting ones stretched legs of the ground. I feel a distinctive difference between those exercises (meaning I can do a reasonable amount of crunches, but can't plank for long) and from what my body tells me this distinction seems to make sense - but it may also be a matter of strength qualities, like power vs. endurance.
On the other hand, sources like exrx.net list both types of exercise as targeting the same muscle and I've never read or heard about the distinction in the english speaking internet or literature.
I want to understand what I read and what I'm told. So, what's happening the truth behind the inner and outer abs?