So, without knowing what the cause of the back pain is, it's very difficult to recommend exercises. You don't want to treat the symptoms, you want to treat the impairment. The back pain could be caused by something in the spinal region - such as a slipped disk, or it could be caused by body mechanics - perhaps you have poor dorsiflexion in your ankle, leading to hiking your hip when you walk, putting pressure on your lumbar spine, or rotating your hip when you walk that is stressing your lumbar region. If I try to treat the back here, I never correct the lack of dorsiflexion in the foot. Just something to consider. I'm hesitant to recommend generic back exercises without identifying the cause.