Finally, pressure research on horses!!!!
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A while back I wrote a post about how much pressure is too much under a saddle, saying that everything I had read was based off human studies, and that I hadn't read anything done on horses. Well, now I have! There was a paper published in the Equine Veterinary Journal Supplement in November of 2010 by K. von Peinen, T. Wiestner, B. con Rechenberg and M.A. Weishaupt called "Relationship between saddle pressure measurements and clinical signs of saddle soreness at the withers". In their introduction, they recognize that human pressure ulcers and saddle pressure problems in horses are different things, and "therefore human pressure models cannot be directly applied to horses". Yes!!! Somebody recognizes this in print!! So, in their study they looked at clinical cases presented to the University of Zurich for "poor performance related to a saddle problem". They measured the pressure on their clinical cases under their normal saddles, ridden by their normal riders, and compared it to the pressure under a group of control horses without saddle problems ridden in their normal saddles by their normal riders. Here's what they found...
The study participants
This was a retrospective study where they went back and looked at the records of horses brought into the clinic between 2006 and 2009 with saddle problems. These were all horses ridden in English saddles, and the problem area was under the headplate where the stirrup bars are attached. They came up with two distinct groups of horses. Group A consisted of 16 horses that had muscle soreness underlying dry spots in this area. Group B consisted of 7 horses that had "saddle sores" in this area. "The area was swollen, warm and very painful to palpation. The skin was traumatised to various degrees." Group C was the control group of 16 healthy horses with no signs of saddle or back problems.
The testing procedure
After the horses were given a full clinical evaluation by an experienced veterinarian, the researchers used a pressure sensor mat under the saddle to record the pressure under the whole saddle as the horse walked, trotted and cantered in a straight line for fifteen stride cycles. Two horses in group A wouldn't canter without bucking, and neither would two in group B. One of those wouldn't even trot. In all cases they attributed this to pain from the saddle. So there was no data from these horses to use in the tests at those gaits.
They took data specifically from the sensors on each side that overlay the area of interest. These worked out to average 4.16 sensors for group A and 3.9 for Group B, so they chose 4 sensors in the same area on the group C horses and used the pressures recorded there to compare. (I would expect there to be unrelenting pressure to some level at this position on the horse, though it would change with movement to varying amounts.) They computed the mean (similar to average but different) pressure over the whole measurement period, and then they noted the maximum pressure recorded over the whole measurement period and compared them statistically.
They recorded the pressures in kPA, kilopascals. Different studies have used different units of measurements, so here are the comparisons:
|1 PSI (pounds per square inch) =
0.689655 N/cm2 (Newtons per centimetre squared) =
51.71032 mmHg (millimetres of mercury) =
6.894 kPa (kilopascals)
What they found
There was a definite, statistically significant difference in the mean pressure on the area under study between all three groups at all gaits. They commented "As seen in other studies Pmean is a more reliable parameter than Pmax because of its higher repeatability."
|Walk||15.3 kPa (2.22 PSI)||24.0 kPa (3.48 PSI)||7.8 kPa (1.13 PSI)|
|Trot||18.1 kPa (2.63 PSI)||29.7 kPa (4.31 PSI)||9.8 kPa (1.42 PSI)|
|Canter||21.4 kPa (3.10 PSI)||28.6 kPa (4.15 PSI)||10.8 kPa (1.57 PSI)|
The results of the maximum pressure recorded were not as clear. Although the control group was always statistically different from the other two groups, it was only at the trot that group A and group B were statistically different from each other. They figured that they could see the differences at the trot because it is the most regular gait and so it produces repeatable results. They also figured that the soreness caused by the saddles in group A and B causes gait irregularities and that would also cause a wider range of variation within the measurements, which they found. However, there was always a difference from the normal horses and the horses with problems.
|Walk||30.6 kPa (4.44 PSI)||38.9 kPa (5.64 PSI)||13.4 kPa (1.94 PSI)|
|Trot||43.5 kPa (6.31 PSI)||53.3 kPa (7.73 PSI)||21.0 kPa (3.05 PSI)|
|Canter||48.9 kPa (7.09 PSI)||56.0 kPa (4.35 PSI)||24.7 kPa (3.58 PSI)|
So the mean pressure under non-problem causing saddles was always under 2 PSI, and usually well under 2 PSI, but even 2 1/4 to just over 3 PSI was causing muscle soreness under dry spots, while 3 1/2 PSI to over 4 PSI caused obvious sores. At the same time, short time periods of higher pressures of up to over 3 1/2 PSI (the maximum pressures recorded at a canter on the normal horses) can be OK, showing again that problems are caused by a time/pressure gradient, not just total pressure.
In comparing minimal prolonged pressures that cause pressure sore problems in human patients (4.26 kPa = .62 PSI) with the lowest level causing problems that they measured with horses with problems (15.3 kPa = 2.22 PSI), they noted a greater than three fold difference, and concluded that "horses tolerate much higher pressure than human patients". They commented that this is probably because the pressure load is transient - the horse is not ridden as long as a wheelchair patient sits in their chair every day - and that the horse is in motion, as opposed to a patient sitting still.
They also commented that shear pressure (which isn't measured with these mats normally) is also involved with pressure ulcers in people, and shear pressure would be high in this area of the horse. So although it wasn't a consideration in this study, it also probably contributes to problems.
They referenced two other articles I haven't been able to get ahold of yet. Neither of them looked specifically at the problem areas (one looked at the whole saddle and one at thirds of the saddle) so the results can't really be compared. However, the one that looked at thirds of the saddle found lower pressures causing "back pain", though how they defined this wasn't stated in this paper. And they didn't mention dry spots or actual sores.
(for whatever they are worth...) This study looks to me to be a very well conducted study that starts giving some solid answers to the question "How much pressure is too much?" under a real saddle on a real horse. It points out the difference in comparing mean pressures (and therefore probably a sustained level of pressure) versus peaks in pressure, and notes that some studies don't mention which numbers they used, making their data rather useless. I would be interested in seeing the minimum pressure in all these scenarios as well to see if that correlated as well. Is the pressure really sustained, or not?
Something more I would like to learn is what level of pressure initiates dry spots, and is that different than dry spots and muscle soreness. Since horses with just dry spots aren't presented as patients to vet clinics, it is unlikely that study will be done very soon, but it would be good to know, as that is a very common scenario saddle and tree makers get asked about.
I find this very interesting. Anyone else have comments?