I don't have back problems but when I started quilting at about 65 yrs old I found that once I got down on the floor it was very difficult to get up!!! I sandwich all my quilts STANDING UP. I bought two 4'x8' pieces of rigid insulation at Home Depot, cut one in half so that I had 6' total width, then duct taped them together.
I start by pinning the backing on the board - wrong side facing out, making sure it is taunt but not stretched...spray baste;
pin on the batting making sure it is straight.. spray baste;
then I pin on the top, I have straight pins on the top of the quilt only at this point. Starting at the top middle I gently smooth down and out, down and out, all the way down the quilt. This is to smooth the quilt and make sure it is stuck to the batting. If you think it needs more spray baste you can raise the quilt, hit it with a few sprays where needed and then smooth out again.
Then I place safety pins about every couple of feet all the way around the quilt just to make sure it stays after I take it down off the board. I usually let it hang for a few hours to let the spray baste dry before I take it down.