From the first writing of horse training in the manual by a Chinese dynasty 3,000 years ago, the horses structure has had to adapt itself to that of a carrying animal. Structurally horses body conformation is a type of prey animals use for maximum running flight. In order to carry a load upon their backs they depend upon the skills of riders who will develop their muscles using careful training regimens. A riders job is to help their horse develop athleticism. Just as each horse is an individual, so too is each rider, each having different ideas about how far to go with her riding and how far to take her training. 
Horses, Athlete or Prey Animal
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You need to finish this article! Not fair to pique my interest and not go on to explain more about back muscles and air bags!
Thank you for your interest in this topic. I put it up for discussion because of the recent news that back muscles are designed anatomically for locomotion not carrying. In my observation, quality of movement is expressed by range of motion. In a riding horse, I find balance is a key factor to range of motion. And that the spine, under the back muscles, is the most important aspect of the body for balance. My point of view is about patterning tension (for lack of a better description). I can explain it this way, when a horse moves and is able to contract only the muscles it needs verus an overall tensing when one muscle contracts and they all do. In the latter case it can destabilize optimum balance because it tugs on the spine unnecessarily. I prefer to see a horse develop using his muscles efficiently as in the former. In no small part because it’s more fun for them and they are happier for it. So the Flair panels in WOW saddles promote circulation in back muscles. As a horse becomes more aware through careful development they can have better, stronger balance, seen as lovely range of motion. And I am just sure anyone reading this would agree, a happy horse makes a human happy!
My horse, and I as his rider, can attest to this. The development of his back has been very exciting, giving us more fun time in the saddle, learning the basics in the correct way. Thank you So Much.