I could not agree MORE with Sahara's point. Absolutely true. The rider also needs to know how to cue the horse to carry itself properly, and know how to be able to feel/otherwise tell when the horse is using his or her back muscles properly.
It takes a fully mature horse a full year of five days of consistent training/week, working on learning to stretch down, build a topline, etc...and most horses are not in proper training to build a solid topline and "use themselves" properly.
I'm having to learn NOW, in preparation for my "future horse", the preliminary aspects of properly maintaining an EXISTING topline on a horse who has been trained and is in good enough shape to carry itself properly. Now, if I wind up purchasing a horse without the knowledge which I barely have, or, without the already existing musculature, then I'll have to pay a trainer not only for lessons for myself, but to ensure I'm learning to do the work with "future horse" to keep him learning correctly AND be able to keep him in proper shape!
This is all just for his proper body mechanics and longevity/health, as again, I have no interest in showing, or "riding under the watchful eye" of any judges... :0)