Here are three basic stretches that will reduce if not eliminate your "bad back".
The theory is simply that tight muscles attached at the lower back need stretching. When they are tense or stronger than their antagonists, they place a stress on the lower back which is where the pain is felt.
For more on the bridge see Matt Furey's book Combat Conditioning. I don't know how an exercise that seems to compress the back can ease back pain, but it does - in minutes!
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