I am currently suffering from sciatic pain in the lower back and right butt cheek, most people would use machines because they think it will help but it actually makes your pain worst in the long term because your body is in an unnatural position, when you have this pain stay away from leg machines and ab crunches, leg raises etc, I start my workout with light cardio, stretching then lower back raises on the floor then different kind of planks this gets me warm up, then I start squatting light and work my way up, the important points, your form has to be textbook perfect if not you will regret it, I still go heavy up to 8 reps on the squat and deadlift and so far I have no problems, dont let anybody tell you that because you have pain you cant squat and deadlift.