Back exercises

My back pain book recommended a stretch in which you lie on the floor, with your hips and knees at a 90 degree angle,  resting your hands on your quads, then lightly press your legs into your hands.  I didn’t even try it, because I knew my back wasn’t strong enough to hold the position.  But when I do the same stretch with my feet resting on an exercise ball, it feels awesome.  I can’t be sure I’m getting the same stretch because the book didn’t include a musculature chart, but it is definitely doing something good.