Not sure of anyone has posted this, but back in the war times in China, they use to tie peoples arms and legs to seperate poles so they were spread out, then plant bamboo seeds underneath them. Apparently bamboo's suppose to grow extremely fast (i heard half a metre a day?), so within days it would be growing through there backs as they would be tied to the floor, and the bamboo would be trying to grow up.
Slow if you create huge cuts in there backs then grow through it, sounds pretty painful to me.