If you have low back strain, in MY OPINION and experience and formal and informal education on weight training and kinesiology, it is probably due to the following reasons:
1) tight hip flexors and erectors coupled with weak abs, glutes, and hamstrings.
2) incorrect exercise pairings (ie: doing too many high stress posterior chain exercises in a workout and throughout the week. I would never hit my posterior chain with more than ONE high stress exercise per week (eg: weighted hyperextensions, romanian deadlifts, good mornings, and weight reverse hypers).
Shoot for one high stress exercise in one session and one low stress exercise (leg curls, bodyweight hypers and reverse hypers, pullthroughs, banded good mornings) in the second session. I would also never do deadlifts in the same session as a high stress exercise for the p-chain.
3) Lack of flexibility in the lower body. This relates to reason 1. Because of the downward pull of the hip flexors, adductors, and abductors the lower back can become filled with tension, forcing the pelvis to impinge on the nerves and muscle there. Read Ian King's articles on this matter.
Before you embark on your next training cycle, I strongly recommend that you undergo a 3 to 4 week corrective phase of training designed BY YOURSELF and FOR YOURSELF. Refer to John Berardi and Mike Mejia's book Scrawny to Brawny on how to do this. I followed their advice for a corrective phase because I was dealing with this lower body issue before the Westside program I am on now.
I plan on doing a 2 to 4 week corrective phase before every 12 to 14 week PL phase from now on. The goals of Berardi's nutrition program are 1)performance, 2) physique, and 3) health. He has stated that when one of these components is not kept up, then the other two will probably go down the drain eventually as well.
This is the same as training. Think of it. If you are not healthy, you are most likely deconditioned, inflexible, tired, and so on. If your physique is suffering, then you lose condition, health, performance (you can't recover as well), etc. And if you lose performance, you can rest assure that your physique will go down the drain as well.
So before anyone thinks they can't engage in some corrective phase because the exercises are sissy style, just think about what a sissy an inury will make you. I hope this helps. Oh yeah, also stretch and massage!