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.
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.
A rigid arrow may be the most efficient tool for flying straight, but it's not made to get around curves. For this, you'd need something that could flex, like a train. At slower gaits, the horse's back muscles relax and become supple, giving him more freedom to bend and stretch.
Ideally, when a horse jogs in a circle, he "bends around a rider's leg." That is, his neck curls slightly, while a less obvious curve immediately below the rider's leg originates in the thoracic spine. Very little bend is possible in the lumbar spine, but the muscles in this area help coordinate a smooth and fluid gait around the turn.
Kinks in the System
Because the spine is at the center of a horse's every movement, any problem that originates there can have repercussions throughout the body. These include elusive lameness, incoordination, paralysis and/or behavioral and training issues manifested as resistance to the saddle, "dipping" of the back when being mounted, crookedness or jump refusals.
Diagnosing a specific source of back pain is often difficult because signs are usually subtle, and individual horses may respond differently to what appear to be identical problems. Sometimes back pain results suddenly after a clear cause--such as a switch to a new saddle, a fall in a trailer or the first lessons in a new, demanding sport. Other times, mild signs may come and go, but build up slowly over months. A horse's back soreness can stem from several sources:
Muscle pain: Hundreds of muscles, large and small, attach to the vertebrae up and down the spine. They are involved in breathing, swinging the tail, extending the head and participating in every step the horse takes. And like any other muscles they are subject to fatigue, major or minor spasms, tears and strains. "In my experience there is a lot of muscle soreness out there that we don't recognize," says Haussler.
In most cases, rest--or a reduction in workload--along with administration of anti-inflammatory medications and/or muscle relaxants will allow back muscles to heal. Some horses may benefit from massage therapy or acupuncture. But it's also important to identify the reason for the sore muscles in the first place. Examine your saddle and pads for fit, and consider adjusting your training schedule.
Fractures: Breaks or stress fractures of the vertebral processes that anchor the muscles involved in locomotion are not uncommon. These injuries can be difficult to detect, but like damaged muscles, they tend to heal readily with rest and medication.
Arthritis: Just like the other joints, the junctures between the vertebrae are subject to wear and tear and inflammation, which can lead to osteoarthritis. Arthritic changes may be detected by ultrasound, but appearances can be deceiving: "It's hard to say if a subtle change is clinically significant," Haussler says. "Some horses can have a lot of arthritic changes but don't seem to be in pain. Others seem severely pained by what looks like more minor arthritis."
Older horses are more likely to be diagnosed with arthritis in the spine but the condition can occur earlier. "I've seen some severe arthritis in younger horses," Haussler says. In addition to judicious exercise, horses with arthritic backs may benefit from anti-inflammatory medications and joint supplements.
Impingement: Another significant source of pain is impingement, when the large dorsal processes of the thoracic or lumbar vertebrae press against each other. Conformation flaws, such as a "hollow" back, can leave some horses more prone to impingement than others, and in many cases, the condition grows worse with age.
Uncharacteristic resistance to being ridden is often an early sign, because the extra pressure of a rider's weight presses the bones against each other. X rays can usually detect impingement; tests performed while the horse is under local anesthesia may help determine the clinical significance.
Some horses with impinged processes never show any signs of discomfort; for those with minor pain, medication and rest can bring relief.
If the pain is more severe, surgery to remove part or all of one or more processes can relieve the pressure on the bones. Most horses recover well and return to work within months, but it's important to rule out all other possible causes for the pain before resorting to surgery.
Abnormal curvature: Horses are subject to three vertebral deformities that cause the back to curve: lordosis, the dipping of the back called swayback; kyphosis, the arch called roachback; and scoliosis, curvature to the side. In congenital cases, the abnormal curvature becomes apparent as the young horse grows. Growth plates within the vertebrae are among the last to close--usually at the age of 5 or 6 years, as the young horse fills out. If a slight deformity appears within the first year or two, it likely will continue getting worse until the vertebrae stop growing. These curvatures may look alarming but in many cases the horses are not nearly as debilitated as you might suppose. With appropriate saddle padding, many can be ridden and lead normal lives.
Wobbles: Cervical stenotic myelopathy, called wobbles, is a malformation of the cervical vertebrae that "pinches" the spinal cord, impairing the horse's ability to coordinate his legs and body. The impingement can be dynamic--if the malformed vertebrae allow too much bend in the neck, putting pressure on the cord--or static, if a buildup of bone tissue causes a narrow spinal channel.
Often associated with young, growing horses, wobbles can occur in any horse, especially an older campaigner, who develops arthritis in the neck or sustains a fracture that causes scar tissue in the spinal canal. Diagnosing "true" wobbles can be tricky: The characteristic sign, progressive incoordination in the hind limbs, can also be caused by EPM, infection or vertebral fractures.
The prognosis for a horse with wobbles depends on how debilitated he is. Wobbles caused by arthritis or injury can often be treated with rest and anti-inflammatory medications. In severe cases of dynamic wobbles, surgery to stabilize the spine may be advisable.
In a condition called spondyloarthropathy, spinal ligaments calcify, creating "bridges" of bone tissue that span the gaps between the lateral processes of neighboring lumbar vertebrae, fusing them together. Because this fusing increases the rigidity in an area of the back that normally has very little flexibility anyway, spondyloarthropathy does not seem to bother horses. In fact, the condition occurs so frequently that it need not be considered a pathology but rather a variation of normal skeletal structure.
If you suspect your horse has a back problem and he doesn't improve after a few days of rest, call in your veterinarian (especially if you notice neurological signs).
The source of equine back pain may be as simple as an ill-fitting saddle or blanket or as subtle as hoof imbalances or hock problems. Arthritis is a common cause of sore backs, as is injury from a fall, collision or awkward buck or kick. A lopsided or rigid rider may contribute to a horse's discomfort, and overtraining--in particular too intense or too frequent repetition of movements or maneuvers--can be hard on an equine back.
In many cases, however, horses will continue to show outward signs of pain with no obvious cause. In that case, "have your veterinarian refer you to a professional who specializes in back problems," Haussler advises. "That can include massage therapists, acupuncturists and chiropractors."
But do some homework, he adds: "Make sure whoever you use is a licensed professional, and make sure your own veterinarian stays in the loop." Haussler also suggests asking whether your specialist has taken continuing education courses from a reputable veterinary organization.
Finally, Haussler says, "Ask what percentage of their practice is horses--100 percent? Or only 5 percent? You want someone who is seeing a lot of equine back pain, because you want someone who is attuned to all of the things that can go wrong."
This article originally appeared in EQUUS 341, March 2006.