The thing about flatwork is that dressage is not just about “teaching” the horse. It is also about getting the horse fit and strong to move in a new way. And usually you want the horse to move in the way he least prefers
so if the horse has a high head above the bit, you work on stretching to the bit. And if the horse dumps on the forehand, you work on gtting him light in front. But the final goal is to get him more balanced on the hindquarters.
Depending on the capability of the rider who is doing the schooling, it can take years to get there very slowly. And I would say even with a very good trainer putting in an honest amount of training rides, it’s going to take 6 months or a year to start seeing a horse really develop the muscles to carry himself in a different way.
If the rider or the trainer don’t really have a clue, then the horse just continues on the forehand with the head rolled behind the vertical until they go lame after 5 years of struggling to do First Level.
The gymnastics I have seen make an improvement in a horse include lateral work inhand, and under saddle walk and then trot, done well before the trot work is “needed” for a test. And lots of attention to stretching over the back, using the hind legs, tracking up. You will see the ab muscles start to develop, and these are what helps the horse lift his back and use himself correctly.
Just going around the ring w t c “ad nauseum” will make no change to the horse’s basic carriage, and if the rider thinks of basic flatwork training as “ad naseum,” then they have missed the point and are unlikely to be creating change or to recognize if it does start to happen.
I complemented one eventer in training on her mare’s impressive ab muscles, and she laughed and said she always worried she was about to colic because of the visible muscles. I think she was serious. Not a clue about what she was doing right, let alone wrong! 