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Effect of weight on the horse's back - Part 1

Posted by RodandDenise on August 24, 2011

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There are many people who teach that the horse rounds his back under saddle so you should build a tree so that it bridges slightly, leaving room for that rounding.  Some even go to the point of poking the horse in the belly to get him to lift his back when they evaluate trees.  This is a highly promoted technique by at least one very influential individual with peer reviewed papers to her name.  There are others who say that you should fit the horse as he stands because they don’t round up under saddle, or at least not in that way, and you don’t want a saddle bridging.  And intuitively, if you put weight in the center of a flexible structure only supported on two ends, it will sag in the middle.  So if you ask tree makers “What would you prefer if you had to make the choice – too little rock or too much rock?” you will get both answers.  A study that evaluates the changes in shape in the horse’s back under tack and weight compared to a bare horse is really valuable to tree and saddle makers. 

Who did it and why?

In 2004, there was a paper published in the Equine Vet Journal written by P. de Cocq, P.R. van Weeren and W. Back from the Faculty of Veterinary Medicine, Utrecht University in the Netherlands.  This university, and especially some of these authors, have published a lot of peer reviewed papers on equine biomechanics and factors that affect them.  This one was titled “Effects of girth, saddle and weight on movements of the horse”.  In it, they state that “the effects of saddle and weight on the back movements of the horse have never been studied…”  We’ve been riding horses for thousands of years, and only within the last 10 – 20 has the technology been available to answer some questions we as saddle and tree makers argue about - probably the same questions the cavalry officers of Alexander the Great were arguing about. 

How did they do the test?

To test this, they evaluated nine horses on a treadmill, at walk, trot and canter.  These were Dutch Warmblood horses all in daily use by the Vet Student’s Riding Association and they were trained to move easily on a treadmill, with and without tack, before they did the test.  They placed reflective markers on their backs behind where the English saddle would go, on the tops of 3 lumbar (loin area) vertebrae – L2, L3 and L5 – and one vertebra in the sacrum (croup area) – S3.  They also put markers on the spine of the scapula (the ridge you feel down the center of the shoulder blade with muscle on both sides of it) and the side of the fetlock joint.  For the hind leg , they put markers on the top of the femur (thigh bone) where it is close to the skin and the side of the hind fetlock.

They have a very sophisticated (and expensive!) system of six cameras that “is based on passive infrared reflective markers and infrared cameras” which take pictures of the markers moving.  Then they crunch the numbers and figure out how the markers all move in all three directions, from which they can tell how the back and legs move relative to the rest of the horse.

So they put the markers on these horses, then got data from each horse at each of the three gaits in four different conditions – no tack, a lunging girth, an English saddle with no weight on it and the same saddle with 75 kg (165 lbs) attached to it.  The weight was solidly attached to the saddle and was distributed with 15 kg (33 lbs) on each stirrup bar and 45 kg (99 lbs) on the seat area. 

What does the marker movement mean?

By comparing the markers on the tops of the vertebra to each other, the researchers could tell 1)  how flexed (rounded) or extended (hollowed or sagging) the back was in each scenario compared to the others and 2) how much total movement there was in the back in each scenario.  Lots of testing has been done before with this system so the methods and basic baselines have been well worked out.

By comparing the markers at the top and the bottom of the legs, they can measure the angles of maximum protraction (how far forward they move the leg ahead of the vertical) and retraction (how far the leg goes behind the vertical before the horse picks it up).  This is important when evaluating back movement because leg movement causes a lot of the flexion and extension of the back.  We know that when a horse has his back hollowed, his hind legs generally don’t go as far forward under the body and do go farther back behind the body.  Similarly, a horse moving with his back hollowed often has his front legs out farther and they don’t go as far under the body before he picks them up.  Turns out there is a cause and effect relationship, but it is the legs that affect the back, not so much the back affecting the legs, although they whole body is connected, of course!  When he moves his hind legs forward, it causes the back the round more.  When he pulls his body far forward over his front legs, he causes his back to round more  (The head affects things too, but that wasn’t part of this study.)  So by measuring these angles they could compare the effects of the tack not only on the back shape and also how the horse dealt that change in shape by how he moved his legs.

What did they find out?

That will have to be in another post.  :)  Stay tuned…

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