I don't know how I did it, but at some point during my empty hand class, I hurt my back. Think of a line going horizontally across the middle of the back (under the shoulderblades), and you have it. I didn't stay for weapons past my bo class and went home. I was OK during my sword class, which means either I wasn't paying attention, or I wasn't using those muscles..

It got worse in the evening, to the point where I was walking slowly, and I couldn't help but play with it a little -- forced to use momentum generated by other muscles in order not to feel pain. Sunday was better but still careful, and Monday I considered not going to work but I felt good enough not to let it stop me. Training on Monday night made it pretty much all go away.

It's very interesting to be in pain, when it's not constant and excruciating (because that just sucks), you can explore better your body's workings.