Ok, I'm not a doctor, but I play one on the internet.
If you're having low back pain - the traditional "I threw my back out" from heavy lifting - do the following (don't laugh).
1. Lay flat on your back - floor or bed.
2. Squeeze your stomach muscles tight.
3. Squeeze your buttocks together tight.
4. Push down with your stomach - like your trying to force a bowel movement.
5. Hold it for a 5 count.
6. Relax.
7. Repeat it as many times as you can up to 10.
I've never had this exercise fail to show almost immediate results. A chiropractor told me about it almost 20 years ago. I also do it as a preventative - if I know I'll be doing heavy work, I'll start doing this about a week before.
As always, YMMV. If you think you've got a serious injury consult your real medical professional or attorney.