I have a really flat back and struggle to do exercise's like, rolling like a ball. My flat back just thuds up and down. I do roll ups with bend knees and use my hands to help pull me up, but my strength and ability don't appear to be improving. Should I avoid these exercises? or are there other modifications, or ways to work up to building up more flexibility in my spine, and being able to do some of these exercises? They feature in the vast majority of videos.