When a horse is poised and directing his mental and physical efforts forward on his line of travel, he is in an athletic state. Until then, even well-chosen exercises will not create many results.
This allows muscle fibers and energy systems to be recruited, taxed, and then recover. The body absorbs the exercise stimulus this way as opposed to just tolerating it. There is a big difference between the body adapting to training on a weekly basis versus going through the motions without underlying physiologic changes.
While dressage— and other arena sports— places relatively low aerobic demands on horses, emerging studies have shown that occasional exercise at higher heart rates may improve muscle function.
How to Keep You Horse Fit Without Riding