We once took in a rescue who needed a lot of work on his topline... This is what our vet told us to do.
Backing up work with HEAD DOWN. If their head is up, they're not exercising the muscles of their back during this exercise. CAVALETTI POLES
, at the trot (start at a walk), make the horse put its head down and pick up its feet higher... Or else it bumps the poles and knocks them over. (The horse has to put its head down to measure how high s/he needs to pick up his feet.)
From there, we set the poles in a circle so the rescue could be lunged over them without the rider getting exhausted (he was in too poor of shape for riding). Healthy back also means healthy belly muscles!
Nutrition is also a must, as already stated above.