PZ you are talking about Sterile Water Injections by the sounds of it, and yes what you have said in relation to them is true. They hurt like a biatch. Punch in the face to distract you from a broken foot, so to speak.

To be honest I don't see why it would be an issue, but I dont think it would help you. You would be better with a heat back, or some rocking on a gym ball, etc.