Yup, riding a horse does make his back extend...
One of the first posts on our blog was about research done that shows a horse's back is extended (more hollow) under weight than when he is just standing in a field. This goes contrary to the commonly taught "rule" that a horse will round his back when he is ridden, or at least when he is asked to collect. Since that study was done with dead weight and not a rider, the objection was that a rider would ask the horse to round and dead weight doesn't. Therefore people could still hold onto their belief that a horse rounds his back under the saddle, despite the reasons given by the authors that a rider wouldn't make a difference. Well, I finally read a study done with a rider that shows the same thing as the one done with dead weight, and guess what? A horse sags under a rider too. Here's the scoop...
How the study was performed
The study was published in the 2009 Equine Veterinary Journal*. They were primarily evaluating the difference in the movement of the back between rising (posting) and sitting trot and comparing that to the motion of the unridden back. They wanted to find out if rising trot really was easier on a horse's back than sitting trot. They also wanted to test the head and neck position to see if and how it changed between the three different situations. The study was done with twelve horses and one rider.
The research group in the Netherlands used sophisticated technology - infrared cameras registering data from special reflective markers placed at specific points on the horse. (Here's a short YouTube video from the Qualisys company whose system they used.) By measuring the angles between markers (a complicated business) they could determine if the back was flexing or extending and by how much. Because the saddle and rider were in the way, they could only measure the angles between certain lumbar vertebrae and not thoracic ones, but it still gave them the information they needed. They also measured angles in the neck, and the height of the head.
So what did they find?
Of course, I was primarily interested in the unridden versus the ridden conditions, and sure enough, the back was always more extended under the rider, both in the rising trot and the sitting trot, than when the horse was not being ridden. They didn't even comment on this much in the paper other than to say that it confirmed what had already been found in the paper I talked about originally. So they weren't at all surprised by this. When I had the chance to talk with Dr. Hilary Clayton in January 2009 and asked her about how a horse's back shape changes under weight, her answer was a very quick and sure "It sags". So the idea of a horse "rounding up under the saddle" doesn't even seem to be a question in the minds of people who actually study biomechanics. They know it doesn't actually happen. It is just one of the many "saddle fit myths" that have been perpetuated without any real backing.
Comparing rising and sitting trot
This was the main point of the study and what they found surprised them a little. The horse's back was just as extended whenever the rider was sitting, whether they were doing a sitting trot or during the sitting phase of the rising trot. They didn't expect that. However, during the rising phase of the trot, the back was almost, but not quite, up to the same level as the unridden horse. So, they learned that the back stays extended more consistently during a sitting trot and is least extended when unridden, but during rising trot there is a lot more variation between maximum extension and maximum flexion. In other words, it has the greatest range of motion during rising trot. (Range of motion was least when unridden and medium in the sitting trot.) The researcher's comments were that they weren't sure if this was good or bad. "It may be more challenging for the lumbar back on one hand (certainly when there is existing pathology in this area) and on the other hand, it may help to create maximal suppleness of movement."
They found that there was more lateral bending happening in the rising trot compared to both the sitting trot and the unridden condition. They made no comment as to why that could be.
They also compared head and neck position in all three conditions. They had asked the rider to be as consistent as possible in where he asked the horse's head to be. (This was English riding, "on contact" with the bit at all times, asking the horse to be in a specific head, neck and body position.) But they found that there were differences between rising and sitting trot. The horse's head in rising trot was lower than in sitting trot or the unridden condition. They also found that while the front part of the horse's neck was more flexed in both ridden conditions compared to the unridden horse, the angle at C6 - the base of the neck - was more extended in sitting trot than rising trot. We know that head and neck positions affect back shape, but it makes sense to me that back position or shape will also affect how the horse can most easily position their head and neck. Either way, we know that the rider's actions cause different effects on the horse, even when they think they are doing the same thing.
So how does this apply to saddles and how they work on horses?
We now know for sure that a horse's back is extended under weight - rider or dead weight - compared to their unweighted, standing position, even when being asked to collect. So the idea of building a bridge into a saddle for the horse "to round up into" is a fallacy. And a tree that bridges a bit without weight is going to bridge a lot more when weighted, very possibly leading to problems. So making sure the back bar tips are off the horse slightly and there is no gapping under the middle of the bars is important when evaluating a saddle for fit.
There have been a few mores studies done comparing different rider positions in different ways. Someday I'll tell you about them too.
*The effect of rising and sitting trot on back movements and head-neck position of the horse P. de Cocq, H. Prinsen, N.C.N. Springer, P.R. van Weeren, M. Schreuder, M. Muller, J.L. van Leeuwen Equine Veterinary Journal, (2009) 41 (5) 423-247