Just finished watching a bunch of videos put out by Doctors of different types and their take on exercises for low back issues. After viewing about 20 exercises from different sources I am not sure if they are trying to fix back problems or CAUSE THEM. Okay so follow me here. Flattening your back to the floor while lying on your back is a decent amount of spinal flexion. Try it against a wall and you will feel how crappy a position you are in. Now somehow doing it while you are laying down is good for you. It is the main position people are in when a disc bulge or herniation occurs but somehow the ground will be good for you. Many people put out videos and give advice with what they consider an understanding of Bio Mechanics but they have lost sight of the Physics principles that they are all based upon. Flexion of the spine may feel good temporarily but what I have seen often is that it causes people to use forward bending and pelvic tucks that weaken overall spinal position in real life. If you are suffering from back pain be very critical about what movements you choose because they just might just be the movements that put you in the hospital and keep you coming back.