The four natural curves of the spine give your back the ability to handle an incredible amount of weight. Are you talking about the end of the float when you body is planked? Normal to have a little inward curve ( lordosis ) when standing tall.I just thought of something worth asking. I’ve noticed that when I allow a slight bend in my back (emphasis on slight) my back feels much better at the end of a set of swings. Would this be allowable?