I have lower back problems myself. I actually just blew it out sat morning. Im just starting to feel better. It is my own fault because ive been slacking ab workout at the gym. I need to get back to it. Plank holds work best as well as supermans. I have a L5 herniated disk and 3 epidermals, and PT. I aslo had the disk pintching my sciatica nerver THAT F'N HURT!. Also make sure hamgstings are stretched out welll, as well as your butt. All this puts stress on your lower back. I hope to be riding again by the this weekend. It is very important to have a strong core for anything not just mountain biking. Your core holds your body together.