One mare I use to ride after tacking up and riding for 45 minutes or so I would have to re tighten the cinch up because she would it seem to let some air out of herself and the cinch would become loose. I always did the finger test on tightening the cinch up. Never seemed to have a problem with her and everyone nicknamed her H*ll-B*tch because of her moods. Also, with your horse she could be stretching and smelling you or trying to get an itch. Check her skin and also your girth for anything that might be poking her or irritating her skin. Could be a sore, or a weed or something like that, don't automatically assume something that you have read online could be the problem with your horse. I tend to find out if I read something and keep watch for it I seem to always find what I'm watching for even if it has nothing to do with what I seen online or read somewhere! Just some suggestions!
Valsalva's Manoeuvre is defined by Elaine Marieb in her internationally used undergraduate medical textbook Human Anatomy and Physiology as "inhaled air being held temporarily in the lower respiratory tract by closing the glottis. The diaphragm and abdominal muscles then contract, and the intra-abdominal pressure rises." (emphases added for present context) ... you can't build up pressure without closing the glottis, which is why people with damage to the recurrent laryngeal nerve who can't reliably fully close their glottis can have great trouble coughing / effectively clearing their airways, and performing other body functions generally taken for granted.
Valsalva's Manoeuvre is naturally performed in mammals during coughing, sneezing, urination, defaecation, labour, and bracing the body when dealing with, for example, pulling or lifting a heavy load. Experienced horses quite conceivably perform Valsalva's Manouevre when anticipating the immediate mounting of the rider, as bracing against the load is helpful for stability. This would explain the observations of some of the contributors to this thread. The breath is held during this process, as explained above. Breath holding is under voluntary control in mammals that are not in immediate oxygen deficit.
I'll add what I think is another reason the cinch gets looser: the saddle shifts to where it is supposed to be instead of where we put it. One custom saddle tree maker uses the analogy of two spoons nestled together:
The curve of the saddle tree has some spot on the back where gravity and the horse's motion will move it. The cinch tends to shift forward or aft to reach the spot of smallest circumference.
To avoid that, I normally put the saddle on with the cinch hanging an inch, then lead my horse through a few figure 8s. That gives the saddle and her shoulders time to get it where it needs to go...a point I now recognize if I use my normal saddle. With most saddles, my cinch or girth needs to angle forward. The sticky neoprene girths I use with my English saddles stay in place well, but my felt (Australian) or mohair (western) ones will slide forward to just behind her elbow. Folks can tell me that is the wrong place for a cinch, but my wool ones end up there regardless.
If you are cinching up really tight to keep the saddle from shifting, you might consider your saddle fit. I'm guilty of using a too wide saddle that I really like and padding up, and that my horse works well with...except for when she doesn't:
As much as I like the saddle, it may go up for sale. Then again, I may keep it in case I ever buy a wider horse. Hmmm....
Last summer, my daughter was cantering Trooper up the trail, weaving from side to side to follow the trail. As they got closer, I noticed something wrong. So I held my hand up to stop her, and then pointed out the cinch had come undone & was hanging 6 inches below Trooper's chest. It was banging Trooper's legs with every stride, but Trooper isn't the sort to panic easily...unlike a certain mare of my acquaintance. My daughter dismounted, retied it, and remounted. But when the saddle fit is really good, and the horse has some withers, you can get by without the cinch. When it works like that, you aren't tempted to offer chicken blood to Norse deities and ask Thor for strength as you go to tighten the girth/cinch.
First off I have done some research on this and have spoken to a Vet in regards to this. Sneezing and coughing cannot be included in this because one cannot do this while holding ones breath. It is NOT the valsalva manuver because air is being past through the lungs, trachea, larynx, nose mouth and it cannot be performed while holding ones breath. Abdominal muscles are being tightened but air is not being held back.
The Valsalva manuver is a unconcious (involuntary) effort in horses and most other mammals (not talking about aquatic mammals)except higher order primates and HUMANS. The Valsalva manure is a concious and unconcious abililty in HUMANS and high order primates. One can indeed hold ones breath WITHOUT abdominal tension this is not Valsalva M.
Horses do not hold the brain power or concious function to perform Valsalva M. Like Humans do. It is a natural bodily function that occurs with out concious effort and is momentary like in defication, labor, urination and etc. A horse does not perform such manuver conciously and is an innate bodily response to the contracture of muscle groups being performed at a given time. He (the horse) does not even remotely even think about it. A horse or other mammal cannot be taught to perform this no more than it can be taught not to. HUMANS on the other hand can be and can perform the Valsalva manuver upon request. The Valsalva M is performed in HUMANS unconciously and conciously during labor,defication and etc.
A horse can and does learn through repition to tighten and hold abdominal muscle upon girthing up but he is indeed breathing. He can tighten his abdominal muscle groups with out having to hold his breath. We humans can do the same thing.
He will not conciously stand there and hold his breath while you figure out what to do with the girth. It goes against all of his basic innate instincts to do such. You can no more teach it than I can teach my cat to dance the mexican hat dance while playing the flute.
This is just another effort of a human being trying to put human concepts emotional or physical into an animal ie the horse. We are NOT the same no matter how much humans want to think that. Thier innate functions are not the same as ours nor is thier brain capacity.
This is the valsalva maneuver as taught by the military for flying:
It is done by attempting exhalation against a closed airway. This is quite different from merely sucking in air and then pausing before exhaling. But it may not be relevant anyways:
"What is interesting is research that shows that the total change in circumference of the chest where the cinch goes is less than an inch, even when the horse is galloping."
But also, as the horse MOVES:
"In a study done on rib cage shape and movement, they discovered that the radius from the center of the horse to the outside of the rib cage can increase over three inches at certain phases of the stride compared to the standing horse. (They think this is because the muscles that pull the body forward toward over the planted front leg attach on the ribs, so that pulls the ribs forward and, because they are curved, out and away from the midline in the same way a bucket handle moves away from the rim of the bucket as you lift it. (1), (2) ) So there really is good reason to use natural material cinches and latigos that have some stretch to them!"
I'm probably not the only person who has gotten down from a horse after 10 minutes and tightened things by a good 3 inches. A difference of that magnitude doesn't come from the horse breathing, holding air in, etc, because the difference is greater than a horse achieves by the effects of breathing while standing.
ZZ: At this stage I recommend an animal anatomy / physiology course, and reading some medical textbooks, and perhaps sitting a few exams on the subjects. Some of the stuff you have posted has some serious misconceptions and self-contradictions in it, which I'm not going to go through one by one. One thing though, in case you're interested: The reason Valsalva's manoeuvre is important in coughing / sneezing is because you have to build up intra-abdominal pressure before you can expel anything effectively, and it's done by closing the glottis, before opening it again to allow the (then more or less explosive) release of air. I don't know where you are getting the idea that horses can't do Valsalva's. All mammals do it. If you are reading and taking on board what some other people have tried to explain here, I don't see much evidence of it, and I'm now going to give up unless I feel my time investment is actually valuable.
I used to teach animal anatomy and physiology at university level, and mark people's examinations on such topics. Feel free to argue with the textbooks if you like.
A horse can and does learn through repition to tighten and hold abdominal muscle upon girthing up but he is indeed breathing.
Fine, split hairs. WhatEVER you call it, horses learn to do this. I believe that are conscious enough and intelligent enough to realize that by doing this the saddle ends up being looser and THEY end up being more comfortable bc of it.
I capitalize on this behavior and encourage it, bc I feel better when my horse is comfortable. Savvy?