I would vote for the strings. But it's not my back.
I know a guy who had his big toenails removed and a line tattoo'd in each nail bed and then the toenails put back. Absolutely true. Why? Because he decided to get a few tattoo's in the most painful places to help him deal with the loss of his father. I think the other locations were shin and ankle, but I was so traumatized by seeing his toe and hearing the story, the rest is a blur. I guess we all have our ways of dealing with things...