This is a very simple stretch that will relieve low back stiffness caused from disc injuries, facet/joint sprains and muscle strains/spasms.  Find a comfortable spot to relax and perform this stretch, allowing your body to slowly curl up from knees bent to up against your chest.  Take your knees as far as comfortable, hold, and perform 5-10 times as comfortable.

If both knees is too uncomfortable because of pain, try just one knee and hold for at least 20 seconds, before switching to the other knee.