ny I can relate - I need to keep the top of my back straight or I get all kinds of neck pain and upper shoulder pain. It isn't natural for me to do that. My fitter said not to suck in my gut but I find if I do suck it in and pull my lower body more perpendicular to the ground I'm getting the sit bones where they need to be and having a straighter upper back. He also said to work on my forward flexibility as that yoga pose appears to help. I need to definitely work on that as my small mileage this spring has me with the stiffness in the neck and shoulders again!