When I think back, even as a perfectly healthy child, I was not comfortable with them. Especially the coming off them bit. Of course, it was easy to correct back then, but it always felt weird. I'm wondering if size has something to do with it - I was growing so fast my muscles could not keep up.

Maybe it's because we don't do it often enough to have our brain get used to it? Moving in a car at 70mph isn't natural either, but somehow we get used to that after a while. Like Elaine says, it does send strange messages to our brain.