I had once intended to write an entire novel while having to urinate very badly. I wanted to see how that need affected the style and tempo of my work. I had found, for instance, that when I'm writing about a character who's in a Ph.D. program and I don't have to urinate badly, I'll have him do a regular three- or four-year program. But if I'm writing a novel and I have to urinate very badly, then I'll push the character through an accelerated Ph.D. program in perhaps only two years, maybe even a year
Patronize your local bookseller and buy any of the quality books produced by Mr. Leyner. In the meantime, you can read a funny short piece by Leyner where he has the Jenny Jones Show discussing serious issues, Alex, The Postmodernist.
I have found it helpful to do the stretching exercises my physical therapist recommends. Do some core strengthening exercises, such as the ones highlighted at The Mayo Clinic. By keeping your core strong, your back won't put itself in a position to cause that sciatica pain you are having.