Some of you may remember the horse that couldn't breathe from one of the past clinics I attended. This horse was carrying tension in its body from a previous traumatic event, and as a result it wasn't breathing properly at the canter - in fact, it was hardly breathing at all. Getting the horse to let go of the tension through movement and breathing was a breakthrough.
Heather was riding a lovely little grulla mare at the clinic. This mare was doing very well in her training, but they were having some issues at the canter. The mare's canter lacked rhythm, and she often had trouble sustaining it. Mark asked if the mare was breathing well, and Heather said yes - she'd worked with the mare in the round pen and the breathing seemed OK.
Mark watched them canter for a while, and said that, although the mare was breathing - she wasn't holding her breath - her breathing was shallow, almost like panting. It was possible to hear this as the mare went around. He said she was carrying some worry about cantering under saddle, and that this was showing up in her tight breathing. And this tight breathing - the tension in her diaphragm - was interfering with her motion at the canter, leading to the choppiness. Which led her to worry about cantering . . . which led to more shallow breathing . . . which led to more worry . . . you get the idea.
The solution was to canter the mare until the breathing came through in a normal one-breath-per-stride manner. They did that on day one, and when the mare came out on day two, the issue was mostly gone, and on day three, her breathing at the canter was completely normal and her canter was no longer choppy or lacking in rhythm and the mare seemed much happier.