Leaning forward when I measure/cut on a standard height table is the worst for me (if I'm sewing at a friend's house or at a guild activity). I use a drafting table at home that's quite a bit higher and is much better for my back.
When I do free motion quilting, I get stiff after awhile, probably because I don't relax doing that....much more focus. So, I have to limit my time at the machine. Taking the dogs for a walk helps.