Driving

Question
Hi,
First off, thank you for your last answer to my question, but another has come up and I would like your opinion.  I have a mini, who when I am working on the ground (lounging, long lining, ect) moves very well.  He is not a pleasure horse with a lot of knee movement, he is more like a western pleasure horse, but he can still flick his front feet up and forward. And as I work him on the ground, I have been working him on his extending trot, not much but I am trying to get him used to what I am asking of him, and he for the most part understands and moves into a big trot.  However, when I started to ask this of him in the cart, once again not much but just so he could learn that this is where he will need to extend, he would trot big a few steps then break into the canter.  Now he did this over and over after I would pull him down to a trot and then ask for another extension.  
Now, when I started to think about this, I begun to watch him move in the cart, trying to figure out what was wrong.  And I noticed that when he is pulling the cart, he doesn't flick his front feet at all.  I know that it is much harder to pull a cart then to move freely, but he moves with almost no shoulder movement when he is hitched to the cart.  I then started to wonder if it was the harness that was restricting his movement.  I watched how the breast collar/strap was fitting him and noticed that it was quite bulky on his chest and seemed to fit directly in his shoulder, (I know that this is what he must push into to pull a cart, but it looked like it was restricting him somewhat).  Then the top of this breast collar/strap, instead of connecting to the saddle, it just goes straight over the top of his neck and when he gets to pulling, it seems to be digging into his thin neck just in front of his wither.  The more I thought about this the more it worried me, because if this is digging into his neck it could be pushing down on his spine, and because he doesn't fight or question anything I would not be able to know that it was uncomfortable for him.  
And I have to say, that he is the smartest horse I have ever worked with, so I have the feeling that he knows what I am asking of him and he is always trying to do what I ask of him, he really likes to please.  I would love to know your opinion on this, should I get another harness, I was thinking of getting a nice show one anyway?  Thanks again!!
Casi

Answer
Hi again Casi-
Thanks for the question. This is one of those that's really hard to diagnose without seeing the horse and the harness. Have you tried ground driving him to see if he has the same reaction? You may be right about the harness fit. Do you have a professional driving trainer that you can consult? Try to find someone who will let you "try" a harness (like test-driving a saddle)or better yet, who will help you fit it to your horse and see if that helps before you actually purchase another one. You might also consult a driving book to compare your fit with what they suggest. It sounds as though you have a nice, willing horse so I would agree that it is probably something else like the harness or the feel of the cart. Good luck. Best, Toni