Last night, Charlie...our adHd (his doctor put the capital H in there) was sitting in front of me while I brushed his hair. He is growing his hair out this summer, and it had gotten tangled while he ran and ran and ran all day, so I was taking my time, and trying to do it gently. After much squirming and moaning, I asked him if the brushing hurt. He said "No... it hurts to sit still!"
I could write an entire post on ADHD, and maybe I will one of these days. But for now, I will just say this. It honestly is physically painful for some kids (and adults!) to sit still.