Why is a vertical forehead - or one close to that not going to be good on the trails? A horse with its head like that is looking straight ahead at where its going... Not from what I've read. A horse has binocular vision over a limited area, and that area when looking ahead is affected by the angle of the head. Thus a horse on a trail, where the footing may be changing constantly - at least where I live - needs to move its head to focus well at different distances. https://en.wikipedia.org/wiki/Equine...n#Visual_field
A horse that's looking upwards and spooks is going to get the action of the bit against the corners of its mouth with no effect and be off rather than on the bars where you can immediately bring them back to you I've never had a horse look UP for more than a few seconds. If the horse isn't too excited, they will respond to the reins regardless. I am a limited fan of curb bits, because I think poll pressure works well for slowing a horse, but a well trained horse will get it done regardless. But when Mia gets excited, it is because she thinks she is racing another horse, and she WILL stretch out like a race horse - which puts the snaffle bit against the molars. Hence I like curb bits...
A horse with a hollow back and hollow neck is far more likely to get spinal problems because its muscle wont support a rider so well Define hollow back & neck. Mia almost NEVER has a hollow neck. She will hollow her back briefly...think of it as collection for the recreational horse. A horse who travels long distances hollow would have problems, and that is usually a sign of a very poor rider or a very poor saddle.
However, all riding lowers the back somewhat. See here for a study: Effect of weight on the horse's back - Part 1
A collected gait doesn't improve longevity or usefulness. The US Cavalry studied it many years ago, and concluded that for travel outside an arena, a collected gait SHORTENS the horse's useful life and HARMS their health.
Most horses are not show horses, and most riders are not 'serious' riders, in the sense of riding 4-6 hours a day or more. I ride about 3 hours/WEEK...maybe 5-6 hours/WEEK when the weather and my schedule permit. A rider who rides at that rate doesn't have the fitness to ride like a top rider, and a horse ridden at that rate won't have the fitness of a competitive horse.
It is fairly easy to train the horse & rider to ride with the horse's back relaxed and free. That is probably as good as a recreational rider, riding a few hours each week, can hope for. The good news is that a horse ridden a few hours each week has less total stress put on its body.
A man has to know his limitations, Clint said, and the same is true of riders and horses. And while a collected horse moving around the dressage ring can be fun to watch, it isn't needed or even possible for the average recreational horse & rider to move like that. We simply lack the fitness and training, and are not going to get it on a recreational riding schedule.
For a recreational rider, terms like collection and particularly headset should be stricken from the vocabulary. We ought to concentrate on what we CAN achieve - balance, looseness, relaxation. For trails and weekend games, that is all that is needed, and really all that can be expected. There is no practical reason to ride a horse with its head tucked in. Folks can do it, but there is a reason you don't see horses run loose like that...