I've had jobs where I had to sit (phone jobs) and was constantly having to bend or unbend my legs because they'd get sore in one position for too long; I'd stand up and "walk" in place for a few minutes every hour (we couldn't leave our desks much at one of those places); at the very least I'd do push-pulls with my arms--you know, push away from the desk, pull toward the desk, using only arm strength--and bob my knees up and down just to get some movement in.  It really is creepy how they can just sit and not move.