This is just for my own learning experience.
A friend has a lovely mare, 16.1hh, pure thoroughbred, raced. She has had the best imaginable care her whole life. When she was younger she was fitted for an endurance saddle, and it fit. Obviously as she's got older she's filled out, now it doesn't. The endurance is semi qh bars.
The issue started when she was on the track. She ended up with a sore back some how, and reacted to it by throwing her head up and wanting to run. She was treated by an experienced chiro and a very experienced equine acupuncturist, and the pain seemed to be gone, but she still has the habit of throwing her head up and wanting to go-go-go all the time. At this point we figured it was a learned behaviour, as she did it in all situations, with various saddles and bits. She was bred, and had 8 months off, and is now coming back into work.
She has been ridden very little, and for extremely short rides this spring, in a couple saddles, but she was showing some mid back soreness, and was due for a chiro check, and the chiro, who is also a saddle fitter, says the endurance saddle is too narrow and its bridging, causing mid back soreness. Why would something putting pressure on the front and back of the saddle cause soreness in the middle?? If it was pinching, would it not cause soreness at the front? If it was bridging, would it not cause soreness at the back?
She then recommended my friend try a treeless saddle, which we're all hesitant on. My experience with them was limited, but I hate how they look and feel, and have heard many stories of sore backs, slippery and rolling saddles, and with my friend doing the same type of riding as me, long fast miles and long days in the mountains, I'm not sure its the best solution.