I hope I am asking in the right section, as it is kind of endurance related, we do endurance. My horse was shoed, had very long toes and low, under-run, collapsed, heels. When he put his hind feet down they would kind of twist to the outside a little right after being put down. He would “hop” into a canter and had lower back pain. So I removed his shoes and got my trimmer (very able, knowledgeable, qualified trimmer) to rehab his feet. All is going very well. His feet look gorgeous. I completely trust my trimmer and he is doing a fantastic job. He is currently in Easy Care glue-on shells. The only thing now is - My horse used to be the most comfortable horse to ride, absolute couch, but now I find him quite uncomfortable. I find it very hard to sit his canter (which is important imo for endurance), and he tires me very quickly, also not a good thing for endurance. Could this be due to the fact that his heels, esp behind, are now more upright (as they should be, angles are almost prefect)? It feels like he throws me up and out of the saddle when we canter, when I come down. If that makes sense. His trot is okay but also not as comfy as it used to be but not at all as bad as the canter. I would love some opinions, thank you.
If he had long toes, no heels, hopped into the canter and had low back pain he probably wasn’t using himself correctly. Now that his feet are in better shape and he’s more comfortable in his body, he is using himself differently. He may need time to learn how to come under himself with his hind legs and you may need to help him.
Changing hoof angles affects everything from the ground up. Put him on a lunge line and watch what he does. New feet, new horse. You need to relearn how to ride him. He may have a longer stride now.
In the gaited world using the trim to enhance the comfort of a gait is very common. It’s also common in the show world to enhance the visual perception of the gait. But the question is more subtle: “Is your horse trimmed to anatomical correctness?” That’s anatomical correctness FOR THE HORSE IN FRONT OF YOU!!!
I did a quick check to find a nice, simple article and found out that this is quite an area of contention! There are several people selling “systems” that, in many instances, are primarily money makers for the seller. I didn’t spend enough time to find an “unbiased” article. Our farriers (mostly pretty good) over the years said you want a phalangeal lever that is short, straight, and a close approximation of the shoulder angle.* This will result in the least stress to suspension system and result in the least propensity to injury.
We had Walkers for a part of this time and in the Walker world the long toe/low heel was favored as it would provide the smoothest gait in a laterally gaited horse. When “tweaked” to provide a more “showy” appearance it could win blue ribbons. The entire system to develop a “Big Lick” horse involved the artificial and dramatic extension of that lever with a stack of leather pads along with shaping the stack to provide the angle that would give the most “lift” and “action” to the gait. Marchador horses are not usually done this way as gait is but one consideration in judging the horses (either in the breed inspection or the show ring) and “messing” with feet is virtually unheard of.
Sometimes when you go back to the native** state of a horse you find out that state is less than desirable. If your horse has a gait that is rough (trot or non-trot) as a result of it’s way of going and conformation then that is the “native” state of your horse. You can accept it or try and modify it. In gaited horses gait quality can very often be improved by just getting the horse into better shape and condition; I’ll presume this can be true in trotters, also. The lateral gait, particularly, is more energy intensive than a trot, including a broken trot. Or the human can turn to artificial methods, such as trims and devices. But what’s done is on the human, not the horse.
So, evaluate the trim and see if it is anatomically appropriate to YOUR horse. Google is your friend and there are unbiased sources out there but you may have to dig for them. If necessary, add a shoe to protect that trim. Or not, as you wish. It’s your horse. But, being your horse, you get the burden and benefit of your husbandry. If, as you work on the problem, you make a bad choice then recognize it, fix it, and don’t do that again! 
We’ve all made mistakes as we learned what is best for the horse in front of us.
Best of luck in your program.
G.
*Among the seven regular farriers we’ve had over the years there was a split of opinion on whether the angle should equal the shoulder or whether there could be minor variations. Nobody every got into a fist fight over it, though!!! 
And, of critical importance, none ever prescribed a “perfect” angle BEFORE looking at the horse. There are multiple systems where “experts” decree that front and rear angles have some very specific values that are independent of the conformation of any individual horse. None of the farriers I regularly used subscribed to this theory, nor did any of the vets I regularly used.
**I use the term “native” vice “natural” because that last word has a LOT of “baggage” associated with it.
The short answer is YES, the trim absolutely can affect the riding comfort of the horse’s gait. Basically, lower (shorter)
heels allow the mechanisms in the hoof and leg to have more shock absorbing flexibilities. When heels are allowed to
become higher and toes shorter, there is not as much shock absorption happening. the key to a proper trim is
allowing the natural shock absorbing mechanisms to happen. Stifling the mechanism long term will cause multiple
problems in the hoof and leg.
It’s impossible to say for sure without seeing before and after video.
But my guess is that your horse is now moving forward more correctly and showing the size of his true stride. Before, he was likely sucking back a little bit and shuffling to some extent because he had back pain. Now he is moving as he was intended to move.
There are two things for you to do.
One is learn to sit his bigger gaits. This would be similar to if you upgraded to a more athletic horse. His gaits aren’t rougher per se, they are just bigger. You need to work on your own seat.
Second is to work on teaching your horse to carry himself, to collect and extend properly. For instance, people don’t sit the warmblood extended trot in dressage the way the horse trots when he’s green. They sit it after the horse has learned to carry his back and the motion turns into more of a bloop bloop bloop feeling rather than a bang bang bang feeling, if that makes sense. I have a Paint with very modest gaits but even she has turned her trot from rattle rattle rattle to bloop bloop as her training has progressed. Also as her canter gets balanced it is much easier to sit.
So it’s a combination of working on your own seat and working on your horse’s ability to balance and carry himself. Also if he has moving constricted for years and is only now starting to move out at his real stride length, then be careful and don’t push him too much as he will need to relearn to use himself correctly.
Thank you for the replies! Will work on our balance and see how it goes but what you say makes sense. I find it very difficult to sit a long stride so I will have to retrain myself also
Maybe you can post pictures of his feet as they are now. Is it possible the new trimmer/farrier has overcorrected
the old problem and his feet are pounding the ground instead of a more natural cushioning stride? Sometimes
it’s helpful to get other’s opinions.
