I tried going through the threads but couldn't remember who said that she were having issues with core workouts on the floor due to stress on your back. I was on pinterest this morning and someone posted an entirely standing ab workout from Jillian, so I thought, for whoever you are, that this might be helpful. (Was it you kwith? This is driving me nuts and I feel terrible that I can't remember!)
Hope this helps with something new-
http://www.youtube.com/watch?v=8zI5EoiYi1c