Before I read this, I was interested in getting the game. Now that I know how regimented it is, I won't be buying it. It seems more of an "on rails" type game, like Manhunt, as opposed to the more open GTA games.
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.