I used to do all of that. It drove people nuts. My elders fussed at me and even punished me all the time until I learned not to do it anymore in my late teens. I could never just sit in a chair. My legs or feet were contantly "flapping" as my mother put it. I'd sit in a kitchen chair at the dinner table and "dance" my upper body. And, I could never just walk anywhere – I skipped everywhere as a child; that was disciplined out of my in grade school.
Incidentally, it was in my late teens that I began to actually get fat. But, I was never particularly thin. In fact, my mother wanted me to be thin and convinced the Navy pediatrician to put me on a 1000 calorie diet when I was around 9 or 10 years old. From the pictures of me at that time, I wasn't skinny like my sister nearest in age; but, I wasn't by any means fat or even chubby. In fact, I wasn't as thick as my mother in pictures of her at the same age. Anyway, that diet kicked of a long, arduous, and ultimately fruitless life of dieting for the better part of three decades – losing a bit, then gaining back more than I lost on endless repeat.