I preach about this all the time, but I had it re-iterated tonight. I'm not a saddle fitter, let me put that out there right off the bat. I've owned Ronan since the end of May, and he's become completely differently muscled since then.
When I bought him, I had two different trainers look at the saddle I had and both said that it looked like it fit really well - and I would agree with them, I felt under the panels and all seemed good. In the 4 months I've owned him, he's started to muscle up differently; a month ago I stopped using a half pad because it seemed a bit on the tight side, and just this past weekend I noticed a rub mark on his shoulder. Recently, he's been less willing to go forwards, and sucks back in his stride. Getting him to lengthen into the trot was a frigging battle.
I had my coach look at it yesterday (long story short, she hasn't seen Ronan in a couple of months) and she was shocked at how poorly it fit, and advised me to stop riding in it right away. Well, I didn't want to lose any riding time, so I called one of our local tack stores who has a master saddle fitter - and he had time to come out tonight. He sat the saddle on his back and was, well for a lack of a better term, appalled. He agreed with my coach on every point she had made yesterday in regards to the fit.
He showed me a million different angles and lines and where muscle needs to be developed and how to achieve this and that... really it all pertains to Ronan so not important information for you guys to read, but basically - the saddle I had (which I adored) didn't fit any longer. Ronan's muscles have been sored by my old saddle. (The saddle itself is LOVELY - it just doesn't fit Ronan.)
He set me up with a saddle (knew right off the bat which one would work, no muss or fuss) and MAN ALIVE. Ronan was a different horse under me. I could feel his shoulder move more freely, his back felt looser, and his hocks started to swing under him properly.
After a proper warm-up, I asked him to move out into his trot ... holy crap. I've never ever felt him move so freely. He lengthened out, and reached into the bit voluntarily. Usually, I'm fighting every stride to get him to move out - tonight he just lifted into this gorgeous, big trot... the canter was lovely as well.
The saddle fits ME properly as well - I can't be sure (coach wasn't there during this trial) but I'm pretty sure that I'm not collapsing as badly through my left shoulder, and I felt straighter.
Really, it's a night and day difference. I'm thrilled with the result - I knew that the saddle didn't fit, but I didn't realize what it was doing to the both of us as a result.
Saddle fit is imperative to your and your horse's ride - don't ignore it!!