I agree that it's the amount of time spent in one position more than the position that causes pain. I used to be able to sew for hours. Now I've found that I need to get up every hour or so and move around or as Prism says switch to another activity. I'll often do laundry while quilting which means I have to get up and go downstairs to put laundry in and switch it to the dryer. I'll stop and fold laundry, too, just to be doing something different. I'm not as productive this way, but I sure feel better.