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Effect of weight on the horse's back - Part 2

Posted by RodandDenise on August 30, 2011

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Last post I told you about a study on how weight on a horse's back affected them, and how the study was performed.

So what did they find…

in the three areas the researchers were checking:
 1.)  how flexed (rounded) or extended (hollowed) the back was 
 2.)  how the total range of motion was affected
 3.)  how the leg positions changed? 

Lunging girth

First, there was no difference at all with a lunging girth, which tells us that pressure around the horse doesn’t affect his back movement.   (There is another study that shows how tight the girth/cinch is does affect respiratory performance.) 

Saddle only

Second, there was no difference with just a saddle at a walk and trot, and only a slight extending effect at a canter.

Saddle with weight

Third, with weight, the back was constantly more extended (hollowed) in all three gaits, but the range of motion was the same as without weight.  In other words, the back moved up and down the same amount, but it was more hollow at the most hollow and not as flexed at the most flexed.  Overall, under weight, a horse’s back sags.  They do not, in and by themselves, round their backs to carry weight.

How much do they sag?

They didn’t talk about this in their write up, but looking at their numbers, the “sag” was 1/3rd to ½ the amount of the normal range of motion at L3 (the third lumbar vertebra in the middle of the loin).  (This would be different at different places on the back because different parts of the back flex and extend different amounts.)  The back moved just as far up and down with each stride, but the “average” position would be 1/3rd to ½ lower than the “average” position without weight. 

How do they explain the saddle only changes at the canter?

They said it “may be explained by the bigger and faster vertical movement of the back of the horse causing a larger acceleration of the saddle resulting in a higher impact on the back due to inertial forces, compared to a walk and trot.”  So, if I read the big words correctly, the saddle bounced up and down a lot more and affected the horse.  Hmmm…  Wonder if a back cinch would make a difference there???  Or if it would be worse with a heavier Western saddle?

Normal leg position effects

Normally, the back rounds more when the front legs are drawn back under the body (the body is pulled forward over the legs) which is retraction.  It extends (hollows) when the front legs are extended out front – protraction.  The reverse is true for the hind.  The back rounds when the hind legs are protracted – moved forward under the body, and hollows/extends when the back legs are extended out behind the body - retracted. 

So how did leg position change?

There were basically changes only in the weight carrying test at the walk and trot, and they primarily affected the front legs.  The legs were retracted – pulled under the horse – more.  Why?  The theory was that the horses were trying to compensate for the weight on their back causing the sag, trying to round their backs by using their front legs.  The researchers guessed that the front legs were used because the front end carries more weight.  There were a couple other small changes at the trot that the researchers put down to the gait being symmetrical with the front and hinds moving at the same time.

There was no change in leg positions at the canter, but the researchers kind of fudged that, saying the data variation was larger, some of the markers fell off so they couldn’t use the data, etc. 

Wouldn’t a rider be different than dead weight?

OK, so this is under dead weight.  What about with a rider who as asking the horse to round up?  They didn’t test that (or didn’t tell us they tried, anyway).  But they mention a couple of other studies I haven’t managed to get my eyes on yet that compare dead weight and riders.  One, testing with force plates that show total pressure from each hoof beat, showed that some weight could be transferred to the hind end compared to the front end with a rider versus with dead weight.  The other, testing movement on a treadmill, found no difference. (I found them!! And I wrote about them here.)  So the researchers felt the dead weight test was adequate to prove the difference. (I found a 2009 study that confirmed that a horse's back sags/extends under a rider too, and wrote about it here.)

So practically, what happens really with a rider really asking a well trained horse to round up?  Is it back to normal standing level?  Not quite?  A bit more?  We don’t know for sure, because they didn’t test that.  For us tree and saddle makers, we do know the horse tries to compensate for the weight with his leg movement naturally but is unable to.  We know that any level of collection is not the “normal riding position” of the majority of horses out there.  Extreme collection is generally held for relatively short time periods, (sliding stops) while moderate collection is not likely, in our view, to compensate more than the standing back, if it even gets there.  But we really don’t know for sure. 

The final story

What happens under weight?  Horse’s backs sag, despite their trying to compensate with leg movement changes. 
How much?  A measureable amount, but not massive. 
What about when asked to collect?  We really don’t know, but that isn’t a constant state for very many horses for very long time periods.

The take home message for tree and saddle makers

Don’t poke the horse in the belly to check out tree and saddle fit.  They don’t normally round under a rider or saddle.  (That isn’t a true mimic of collection anyway.) 
Fitting the standing back isn’t out of line, knowing the changes, while measureable, aren’t huge. 
Make sure the front and back bar tips have lots of relief built in so they don’t poke the horse when he sags a bit. 
And if you’re asked if you want a little too much or a little too little rock – go for the extra.  Poking the loin and shoulders is a lot worse on the horse than the back end of the saddle lifting off a bit, decreasing surface area and increasing the Pressure a bit.

PS.  I wrote a follow up post about this study which you can see by clicking here:  Of sagging backs and tickling tummies

Comments:

Posted by Christine C. Macdonald on
Hmmm...interesting! Thanks, chris
Posted by RodandDenise on
Yes, Chris. I find it interesting too. It sure isn't what you commonly hear when people talk about horse's backs, but when you think about it, it is logical. And ever since I read this I've been watching, and you can see the change in a horse's posture as people mount and dismount. They do sag. It is a lot harder to visualize when the horse is in motion. But looking at where this study was done (Europe) and the horses used (Warmbloods) I would expect they were primarily used as arena horses and probably dressage trained. If that is the case, they would have been taught specifically to collect (as compared to a lot of our recreational horses that are primarily trail ridden). So if they found that set of horses sagged during movement (compared to standing), I would expect it really is the case.
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