My hubby really wanted to make sure this was sturdy and stayed put on the wall, so he opted to use a couple of long wood screws to attach the whole shelf to the wall.
Stay away from crunches or anything that puts you in a crunch - like position (e.g., v - sits and oblique twists) as these will increase the pressure on the abdominal wall and pelvic floor and create too much load on the spine.