The conversation re where to ride a horse is something that I find to be fascinating. The absolute to me is that you ride the horse in the posture that is best suited to get to the back and the hind legs. If you don't have the back and you don't have the hinds, you have nothing that will gymnastically develop the horse over time. With my boy, I was being a bit militant about riding him poll high because in my mind, that is classical and that's where it has to be. However, an interesting thing happened as he grew and his neck got thicker and he developed. I lost submission, and I didn't like the feel in his back. So, I put him a bit lower, which made him a bit rounder. Did that shorten the neck for a bit? Yes. However, was I able to get to the hind legs? Yes... because I wasn't hollowing his back, and submission improved, so I was able to continue to shape him. Now, I can ride the neck back out. So, in my mind, it's a valuable lesson... poll high may be the ideal - but, sometimes, based on specific training issues, or specific conformation, it's OK to go lower and rounder. (please note, I am NOT talking about rollkur or any bastardizations thereof.) Had I continued to ride him with the poll at the absolute highest point, and not listened to what his body was telling me, I would have stalled progress quite a bit.
Also, submission seems to be a bad word, because of its negative connotation and tie to rollkur. However, imo, the horse WILLFULLY submitting it's body and mind to the rider is intrinsic to dressage. If you don't have submission to the hand, to the leg, to the aids, you cannot shape the horse. So, if submission is taking a hit, you have to change something to get it back (flexions whatever). Because again, without submission, especially to the hand, you cannot access the back and the hind legs of the horse because you cannot take an effective half halt to shape the energy.
Now, in regards to the forelegs - you can absolutely teach them to reach through the forelegs - but you have to push the mediums to the point of breaking into the canter - because just before they break - they will hit the limit of the stride. The goal is NOT to have them pass the tipping point, but, it teaches them to use their bodies in a different way. It may run a bit, it may be a bit choppy, but again, just before they hit the point of breaking into canter, you will feel that open up the last bit. To me, it feels like they suddenly roll through the shoulders like a swimmer does. I've seen a number of horses with rather limited reach be trained to have very, very respectable medium and extended gaits by doing this.
And, just for fun... canter vid and a wee bit of trot:
https://www.youtube.com/watch?v=gCtHFCaWfXEhttp://www.youtube.com/watch?v=XZvhg2hNF-E