Zuzu, my back cramps up after about 10 minutes of any sort of stooping, or other weeding-related motion. If I overdo it, I know a hot bath is the way to undo the spasms. Medicine doesn't work at all unless I take a muscle relaxant and that puts me straight to sleep for 12 hours . . impossible.
I solved the problem of being able to weed by sitting on a big exercise ball - you know, the kind they have at the gym for doing abs and such. Wal-Mart has them for about $10 to $18 depending on what size you want. It's soft and comfy and I can roll it along on any surface to move. I have two sizes, in fact, a 20in. and a 30 in. but I can't use the smaller one for very long because I'm too scrunched down. The big one lets me weed in comfort for quite a while.
Tell you the truth, I still don't do nearly enough weeding, but when I do I'm comfortable, both then and afterwards. It helps to gently stretch out after I get up, too. I don't have the winter hiatus that you do, my garden needs weeding year 'round, sadly. But it probably keeps me in shape a little bit to do it.
