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As dressage riders we are well aware of the importance of the movements of the horse's spine. As a researcher I'm always on the lookout for new information on spinal mechanics. Currently, at least three research groups in Europe are working on the horse's back and in this column I'll share some of their results with you. First, though, let's review the anatomy of the horse's spine. Horses have approximately 56 vertebrae form a supporting framework for the neck, back and tail. There are 7 cervical (neck region), 18 thoracic (withers and saddle region), 6 lumbar (loin region), 5 sacral (croup region) and about 20 caudal (tail region) vertebrae (see diagram). A small amount of movement is possible at the intervertebral joints between adjacent vertebrae, and the summation of these small movements determines the overall shape of the horse's neck and back. The amount of motion varies greatly in different regions.
The horse's neck is quite mobile in all directions, as it needs to be for grazing, self-grooming and other behavioral postures. The other mobile region is the tail, which is both an active fly swatter and an indicator of irritation or discomfort. The 'back' which includes the thoracic, lumbar and sacral regions, is designed for rigidity rather than flexibility, which is necessary to provide secure attachment of the fore and hind limbs and for transmission of the propulsive forces forward from the hind limbs. Although there is only a small amount of flexion and extension at each intervertebral joint, this adds up to an appreciable amount of movement of the horse's back as a whole. We're going to concentrate on flexion and extension of the intervertebral joints which produce rounding and hollowing, respectively, of the horse's neck and back. Studies of horse cadavers have measured the maximal range of flexion and extension possible at each of the joints of the equine spine and these are shown in the graph. The atlanto-occipital joint, which is between the skull and the first cervical vertebra, allows flexion and extension at the poll through a range of almost 90 degrees. The joints further down the neck are all quite flexible, allowing 15 to 35 degrees of motion, which is a great deal more than we need to achieve roundness of the neck. The first interthoracic joint, just in front of the withers, has about 10 degrees of motion, but the rest of the back is much less mobile. In the withers there is minimal motion, no more than about one degree at each joint. This is because the withers provide stability for attachment of the fore limb and for transmission of the forelimb forces that raise the withers and the forehand as a whole. In the region under the saddle each joint has 3 to 4 degrees of flexion and extension, which is enough for us to see and feel the effects when the horse rounds and hollows his back under our seat. The lumbar region, just behind the saddle, also has 3 to 4 degrees of motion at each joint. The lumbosacral joint, between the last lumbar vertebra and the fused sacral vertebrae is atypical in that it allows considerably more motion, about 30 degrees of flexion and extension. This joint allows the pelvis to rotate forward bringing the hind legs underneath the horse, which is so important in collection. Sometimes movements in one part of the horse's spine are correlated with movements in another region. An example of this is that when the head and neck are lowered, the back is rounded and, conversely, when the head are raised the back hollows. To maximize the rounding effect in the horse's back, the outline of the neck should be low and deep (flexion of the cervical region) rather than low and stretched forward (extension of the cervical region). This explains why working the horse in a deep, round frame is beneficial for stretching the topline of the back. As a horse trots naturally without a rider, the middle part of his back oscillates up and down twice in each stride. It rounds during the suspensions, then hollows a little as the diagonal limbs are weight-bearing. The lumbosacral joint also oscillates in rhythm with the stride, it is maximally rounded as the hind limbs reach forward, and then flattens as the hind limb pushes back during its propulsive phase. Assuming the horse's back continues to swing in the same manner when he is being ridden, the area under the saddle comes up to meet the rider's seat during the suspension, then tends to flatten somewhat as the rider's weight is descending. The muscles control the back movements in such a way that the flexor (abdominal) muscles are active as the back extends and the extensor muscles (muscles along the top of the spine) are active as the back flexes. At first this sounds contrary to what you would expect. What this tells us, though, is that the movements between the vertebrae are created by the forces of locomotion and are controlled by the back muscles. In other words, it is not the back muscles don't cause the spine to round and hollow, instead they control the amount of rounding and hollowing. What we need to know next is how the movements of the back and the actions of the muscles change during training as the horse learns to move in a rounder outline, and which muscles are used to create collection and self carriage. |
