Anecdotally, I do a lot of exercise and for a year I'd always wake up with a sore back (top of hips, either side of spine), or after sitting down. Physio eventually told me my hip flexors were really tight and that was the cause of the referred pain. 4 days of long stretches on those a couple of times a day and I'm good as! Crazy I went for a year with it irritating. If it helps someone, glad it did. Something like this (first result on google) http://www.stack.com/a/4-hip-flexor-stretches-to-relieve-tig...
