Try lying on your front and do a sit up in reverse. The weight of your upper body is enough and helps strengthen your lower back. Ironically it was an osteopath

Try lying on your front and do a sit up in reverse. The weight of your upper body is enough and helps strengthen your lower back. Ironically it was an osteopath who told me of this exercise as I used to have lower back pain. After a few weeks doing it I never needs to go back! In addition, running and sit ups can also help massively. But don't over do it until you're completely mended.