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Saddle fit - How much pressure is too much?

Posted by RodandDenise on December 18, 2011

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When you read about saddle fit, you find numbers quoted as to how much pressure is acceptable under a saddle and how much is too much, and those numbers will vary dramatically.  Why?  I figured we should know by now how much pressure will cause damage to underlying tissues, so I went looking for answers.  Turns out there are no solid answers and really, this isn’t even the right question.

No research on horses

I have not yet found any published research that has been done on horses' backs, or horses at all.  Maybe I just haven’t come across it yet, but by now I doubt that any exists.  (Since I wrote this, I have found one article about pressure research on horses.  I discuss the study in this post.)  All the numbers people quote, if they list a source, have come from studies in humans where pressure ulcers (“bedsores” in common language) cause suffering and even deaths every year, not to mention the costs involved in treating them.  There are international organizations and symposia just to deal with the problem of bedsores – and even they don’t have answers to “how much pressure is too much?”  Nor do they fully understand how pressure actually causes the damage at either the tissue or the cellular level.  In learning this, I started to realize that the problem is a lot more complicated than I had thought.

A combination of time and pressure

In 1976, JB Reswick and J Rogers wrote a paper where they presented a graph (you can see it by clicking here) that gave their best idea, based on older research and their clinical experience, of how much pressure for how much time was acceptable or unacceptable in terms of causing pressure sore problems.  While they said this should serve only as a guideline, it has been called “one of the most important papers in the history of pressure ulcer research”.  It laid the groundwork for the idea that high pressure over a short time could do as much damage as low pressure over a long time.  And in thinking about it, this makes perfect sense.  A hammer hitting your thumb isn’t there long (even though the black nail stays for months!) but it does damage, while a patient lying on a soft bed with very low PSI under them will get a pressure sore if he isn’t moved occasionally.  So absolute pressure isn’t the only factor to consider. 

“Smart underwear”

In October, 2009, I talked to a researcher in the human biomedical section of the University of Alberta.  They are working on Smart Underwear for people immobile in wheelchairs.  What they have found is that a few seconds of electrical stimulation to buttock muscles every few minutes will get the muscles to contract and the blood flow moving enough to prevent pressure sores.  This is true even with very atrophied muscles that have no normal nerve stimulation such as are present in paraplegics.  This looks to be a great innovation when it finally gets used in practical situations.

Applying this to saddles

So if a few seconds of very minor muscle movement every 10 minutes or so will prevent pressure ulcers over bony prominences (the worst places) in situations of long term, total immobility where the patient can’t feel the pressure, why do we still have damage to active, moveable muscle, not over points of bone, on horses that can feel, shift and move?  There has to be more involved than just a certain level of pressure.  And if pressure alone isn’t enough information, but pressure and time together work inversely to cause pressure sores, why do we read and hear numbers listing maximum acceptable PSI under a saddle as if these numbers are an absolute law that must be obeyed?  Simply because we don’t know enough to know what we don’t know.

So are studies using pressure sensor pads useless?

No.  Finding out how much pressure is under a saddle gives us all sorts of information.  It can let us know where high pressure areas are – so we can figure out why.  It shows us what really happens under a saddle when you change things – so we can figure out how to make changes that won’t cause negative results.  It can tell us if our theories about saddle fit are right or not - because when we compare between known variables, we can see how they affect total pressure and point pressure.  So pressure sensor pads are a valuable tool that can be used to research saddles and their fit.  But to use a pressure sensor pad and say “this saddle won’t work because the pressures get over (pick your number) PSI at any one time?”  Nope.  That is based on false information.

So the next time you hear someone giving specific numbers as authoritative fact, you may want to question their other statements too.  If the foundational “facts” are false, the conclusions based on them can’t be trusted either.

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