The horse is really an extraordinary animal. Weighing in at over a ton, he is capable of jumping an obstacle of over 2 m (7 ft.) high and over 6 m (20 ft.) long. He is also capable of travelling at over 64 km/h (40 mph) for a mile, and over 16 km/h (10 mph) for a hundred miles! He is all the more extraordinary because he can do all this whilst carrying a human being weighing up to 20 percent of his own body weight. No other creature comes close to this performance in harmony with man, which is why the horse has played such a major part in human history.
It is uncanny how much suited a horse is to being ridden. The saddle goes on the narrowest part of the animal and where the rider's calves or heels rest there are motor nerve connections to the horse's hind legs, so the horse naturally goes forwards when the rider's legs are used in this area. It is important to remember, however, that even though the horse is well suited to the purpose, he was not designed to carry a human being and our challenge is to find ways to allow this burden to be carried easily. The key to the ridden horse's comfort is an understanding of how he is built.
The horse's spine is a fairly rigid structure which is necessary to support the heavy body and cope with the huge forces exerted through the hind legs. The vertebrae under the horse's croup (near the tail) are fused and those from the withers (shoulders) to the croup are capable of only limited movement. However the small arching and sideways movement that is possible in the area under and just behind the saddle is all-important. The chain of ligaments and muscles stretching from poll to dock work like a suspension bridge with a main support at the lumbro-sacral joint (just before the croup) and a subsidiary support point where the neck meets the withers. The floor of this bridge is slightly curved and a pull at either end of the muscular chain will have the effect of raising the back. By far the most important pull is provided by the horse engaging his hind legs. This, in combination with the contraction of the belly muscles, creates a ring of musculature to bring the back up to form the desired arch.
Training the horse to adopt a rounded outline is important because it helps him bear the weight of the rider without placing excessive stress on his spine. If a horse's back is not arched, his useful life will be shortened and his mental attitude may be harmed.
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