Horse losing muscle/fat over withers/first couple thoracic vertebrae?

My 17 year old mare has the exact same thing going on, except we do different disciplines. I'm not sure if it's normal considering what you do, but I'm trying to get more muscle in that area with my mare, I heard somewhere if they don't have enough muscle there it can cause a sway back, my mare has a pretty high wither already so I want her to get more definition along her topline. Anyway, a good exercise is to get them to ride up hills.