I too had problems with quilting for long periods of time and muscle stiffness. Then I found the cure (or at least help).
Every hour, stop and do the following.
1) nod your head and say Yes, (5 x)
2 ) turn head from side to side(chin to shoulder) no(5 x)
3) lift shoulders up and down (i don't know) 5 x
4) get up, walk to kitchen, get a drink (your choice)
5) start quilting again.
Hope this helps