OK, trying to start a discussion here instead of just writing an article like I did accidentally the last time I started a thread. :)
I'm a very logical thinker so I am trying to understand something, not just argue for the sake of it.
I am having difficulty with this concept: an arched neck and head on the vertical is necessary for collection. I understand the concept of collection and that the theory is that part of getting a rounded, strong back under saddle includes the stretched vertebrae in the neck. I've heard that "an arch is strong," and therefore helps the horse carry a rider better. But I am not convinced this arched back is only possible when a horse also has the neck vertebrae in an arched position.
We supposedly improve horses by training them and riding them. I only ever see loose horses briefly travel in the proper dressage form, but I see some naturally collected horses that travel from their hind end with their front end elevated and they are collecting and extending beautifully on their own without arching their neck or placing their head on the vertical.
I keep seeing photos of horses and horses in person that are running free with their neck up but not arched and their head well in front of the vertical, but to me their backs look rounded and they are self-collected.
There are also so many disciplines that use horses "hard," where the horses seem collected but never strive for the dressage form.
I can round my own back and arch my neck and head back at the same time.
As for under saddle: endurance riders can ride for 50 miles or more without their horses getting a sore back yet most of them don't ride with their horses necks arched down or on the bit. Their horses carry a lot of muscle and have strong backs. If the dressage position is the best for the horses' back, then why do the people who ride the farthest ride in a different frame? Endurance horses usually assume a frame more similar to a horse that is running free, with head and neck high but not in a sustained arch. While endurance horses are usually more in extension than collection, dressage riders still keep the arched neck and head on the vertical when they are extending gaits. The endurance horses have to be using their bodies properly or they wouldn't remain sound for so many miles, right?
Please help me try to understand this.