Your Horse's Back
Ever notice differences in how animals move their bodies?
Your dog comes running up, his entire rear end swinging back and forth almost as fast as his wagging tail. Then he drops his hips and scuttles up against your legs before rolling belly-up to wriggle at your feet.
Your cat dances in circles on her toes, head up and back arched high, then walks a serpentine around and between your legs; you lift her and she goes limp, almost as though she has no bones--until she spots a squirrel and launches herself like a coiled spring. The squirrel, meanwhile, races up a tree then sits up on its hindquarters, craning and bending to keep the stalking cat in view.
Your horse’s body, in contrast, seems as solid as a rock as you brush him, and he doesn’t flinch as you place the saddle securely behind his withers and swing onto his back. You feel only a gentle undulation as he moves off.
As diverse as they may seem, mammals have more similarities than differences in how their backs are constructed. All share the same basic lineup of bones called vertebrae, which enclose the spinal cord and support the skull, ribs and limbs.
Yet form follows function throughout the animal kingdom. Obviously horses don’t need to climb trees or bring down prey. Instead they must be able to cover ground efficiently and, when needed, sprint away from predators. In fact, horses in their natural environment often travel dozens of miles a day to reach watering holes or better grazing, sometimes sustaining a trot for hours.
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The horse’s spine is designed to aid efficient locomotion. In biological terms, that means getting a maximum return on a minimal input of energy.
“Horses’ legs have developed special tendons that act like big springs with little muscular involvement at the trot,” says Kevin Haussler, DVM, DC, PhD, assistant professor of equine anatomy at Colorado State University. “Treadmill studies have shown that the back doesn’t move much at that gait; it has the ability to become a passive structure. The trot is a very efficient gait when you need to travel 30 or 40 miles. In contrast, there’s much more flexion at the canter. That gait is best for short emergencies.”
In the modern world, of course, a horse’s work is more varied, and in many sports he is asked to perform at the extremes of his gaits while carrying the weight of a saddle and rider. And even the trail horse, whose life may be less demanding, is still at the mercy of his rider’s imperatives. As well-engineered as a horse’s back may be, these demands can strain the system.
With a basic understanding of how your horse’s back is put together and how it works, you’ll be better able to spot potential problems before they produce any chronic aches and pains or worse--and that will help keep him sound, comfortable and happy for years to come.
Slings & Arrows
A horse’s spine is like a suspension bridge, a connective structure between the uprights of the front and back legs, rigid but somewhat flexible, capped off on each end by the highly mobile neck and tail.
All vertebrae share several characteristics. Viewed head-on, the most noticeable feature is a hole in the middle for the spinal cord. In addition, all have bony projections on each side, called lateral processes, and a single projection on the top, called the dorsal (spinous) process. And holding all of the vertebrae together is a system of muscles and ligaments.
A horse’s spine operates much like an arrow shot from a bow. When the string is released, the arrow flies fast and straight. But imagine shooting an arrow made out of Styrofoam; the soft shaft would wobble so much, it’d be lucky to get near the target, and if it did, it would most likely snap on impact. If your arrow were curved, it would take a great deal more energy to get it to travel in a straight line.
In much the same way, a horse’s hindquarters propel him forward at the gallop. The bend at the lumbosacral joint allows him to bring his hind legs way up under him, to maximize his thrust, and as he gains speed the muscles supporting the thoracic and lumbar vertebrae tighten, turning his spine into an even more straight and rigid projectile. Any bending or curvature between the withers and rump would reduce efficiency; there is only enough flex to absorb the forces of impact with each stride. Meanwhile, the rhythmic rise and fall of the neck acts like both a pendulum and a flywheel, assisting and smoothing the momentum of the body, coordinating the strides of the forelegs and helping the horse maintain his balance.
