Have you hefted an average school-kid’s backpack recently? Years ago, when some of us were in school, we carried maybe two or three textbooks at a time. Nowadays, however, with many schools eliminating lockers for security reasons, students often carry all of their materials, all day long. One 2004 study of 3,498 middle-school students found an average backpack weight of 10.6 pounds, with some ranging as high as 37 pounds. Not surprisingly, 64 percent of the kids said that they’d experienced back pain, which correlated directly to the amount they carried. That is, the more the backpack weighed, the greater the likelihood the student would report pain.

In response, several health organizations advise that student backpack weight be limited–the American Chiropractic Association suggests that kids carry no more than 10 percent of their body weight, and the American Occupational Therapy Association recommends 15 percent. If equivalent guidelines were adopted in the equestrian world, the loads placed on a 1,000-pound horse would be restricted to 100 to 150 pounds.

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Of course, horses routinely bear far heavier burdens without apparent difficulty. But that doesn’t mean that there’s no cost. Over the past few years, researchers at the California State Polytechnic University in Pomona have been investigating the range of physiologic changes that occur in horses when they carry varying loads. “Our studies dealt with energetics, to quantify the costs of carrying weight,” explains Steven Wickler, DVM, PhD, who headed the research team. Among the areas investigated were how weight affects equine biomechanics, metabolism and potential soundness.

Although this research has direct implications for elite equine athletes–particularly in such sports as racing or endurance–Wickler emphasizes that his findings potentially have much broader implications, extending to recreational trail mounts and backyard horses. “Look at the American population today,” he says. Over the past few decades the U.S. population has on average been getting taller and heavier, and the number of obese people is increasing, according to the National Center for Health Statistics. “If you take a 220-pound person, add in a Western saddle, plus everything else you carry, then head out for a whole day on the trails, you could be stressing that horse quite a lot.”

Exactly how much weight is too much? The answer is still, largely, “It depends.” But an increased awareness of weight issues can go a long way toward keeping your horse healthy and sound for years to come.

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Loaded Questions All creatures in nature perform a delicate balancing act. On the one hand, they need to carry a complete set of survival tools–the muscles they use to sprint, leap, fly or climb out of harm’s way; the hoof, horn, tooth and claw they need to fight their battles. On the other hand, growing and maintaining those tools requires energy, which must be derived from available food resources.

Because of the metabolic costs associated with maintaining their bodies, animals tend to pack just as much muscle and bone as they need, with only a little leeway for emergencies. “Human engineers will overbuild to anticipate extremes,” says Wickler. “For example, an elevator may be built with a posted capacity of eight people, or no more than 1,500 pounds. But, in fact, that cable may actually be capable of holding 15,000 pounds–that’s a safety factor of 10. But biological systems don’t do that. Biologicals have a built-in safety factor of around two.”

When a horse carries a rider, it is this “reserve capacity” that handles the extra weight, but the horse must nonetheless adjust the way he moves and uses his muscles to accommodate the load. The Cal State researchers have quantified some of the ways added weight changes the way equine bodies function. Here’s what they’ve measured:

Metabolism “We expected that when you weight a horse, metabolism would go up in direct proportion, based on comparative literature in many animals, including humans,” says Wickler. Researchers measured the amount of oxygen horses utilized as they trotted on a treadmill wearing face masks. As the horses were tested at different speeds–low (5.05 miles per hour [mph]), moderate (about 7.4 mph) or high (10 mph)–the amount of oxygen they used also increased. When weights were added that equaled about 19 percent of body weight, an amount that is roughly equivalent to a 150-pound rider plus tack, the horses’ metabolism increased by an average of 17.6 percent at all speeds.

“The increase in your metabolism is directly proportional to the increase in the weight,” Wickler explains. “So if you add 10 percent of your body weight, your costs go up 10 percent.” Each additional pound added to the load produces a corresponding increase in the metabolic effort required to move that load–and that’s over level ground. “If the horse is asked to trot uphill, metabolism increases. For a modest grade, metabolism increases by 2.5 times,” Wickler adds. “Over the long term, this work can be equated with calories and an increasing need for nutrition.”

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Economy Not surprisingly, horses who are free to choose their own speed tend to slow down when weight is placed on their backs. Coupled with their investigation into metabolic changes, says Wickler, “we were also hoping to see a tie between metabolic economy and preferred speed.”

In this phase of the study, seven Arabian geldings and mares were trained to walk and trot along a level fence line in response to voice commands. They were timed as they walked and trotted the distance unburdened as well as with a saddle weighted with lead shot. The saddle and lead together weighed 85 kilograms (about 187 pounds), which amounted to about 19 percent of the horses’ body weights. Not surprisingly, the additional weight caused horses to move more slowly, reducing speed from about 7.4 mph to about 7 mph. “Not only does their metabolic rate go up, but their preferred speed goes down,” Wickler says, adding that the most important finding was that the horses’ preferred speed was the most economical in terms of moving a given distance with that added weight.

Forces on Legs Increasing the weight a horse carries also increases the ground reaction forces–the amount of energy that “pushes back” on the sole of the foot when it strikes the ground–that each limb withstands with each stride. “When you add weight when a horse is standing, the force of the weight is divided through all four limbs,” Wickler says. “But as he gallops, not only do the forces go up, but also at different times throughout the strides all of the weight must be supported on some limbs individually.”

To find out how horses compensate for these changing forces, seven horses–four Arabians, two Thoroughbreds and one Quarter Horse–were trotted at a range of speeds across a force-measuring plate both on the level and at a 10 percent incline. Normal (vertical) and parallel (horizontal) forces as well as each foot’s time of contact on the plate were recorded on the fore- and hind limbs; each horse was also videotaped so that stride time could be measured.

Because a trotting horse looks like he is using his diagonal feet in perfect tandem, it might seem as if the reaction forces would be evenly distributed across the two legs that support him at each phase of the stride. But in fact, there are significant differences in the amount of forces borne by the front and rear legs. On a level surface the forelimbs consistently supported 57 percent of the forces while the hind limbs supported 43 percent. Going uphill, this pattern of distribution shifts, with 52 percent supported by the forelimbs while the hind limbs took on 48 percent. Time of contact also varied. At higher speeds, the two feet were on the ground about the same amount of time, but at slower speeds, the hind limbs tended to spend less time on the ground–an observation that had never been made before in quadrupeds, according to Wickler. For the front limbs, time of contact didn’t change significantly whether on the level or on the incline, but the hind limbs tended to be in contact with the ground longer when going uphill.

Gait To study the biomechanical effects of loads, the Cal State researchers trotted five Arabians at a consistent speed on a treadmill under three different conditions: on the level with no load, on a 10 percent incline with no load, and on the level while carrying a saddle and weights that totaled about 19 percent of their body mass. To record the motion and speed of the horses’ foot movements, an accelerometer was attached to the right hind hoof, and the sessions were recorded with a high-speed video camera.

Carrying a load caused the horses to leave their feet on the ground an average of 7.7 percent longer than they did while trotting unburdened. On the level, the addition of a load caused the swing phase of the stride to become 3 percent shorter, but going uphill this phase of stride lasted 6 percent longer.

In short, explains Wickler, carrying a load causes a horse to shorten his stride, leave his feet on the ground longer and increase the distance his body travels (the “step length”) with each stride. All of these gait adjustments work together to reduce the forces placed on the legs with each step. “Forces are damaging,” says Wickler, “so keeping the foot on the ground reduces peak forces and reduces that potential for injury.”

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Tough Road? All of these shifts in how horses carry themselves in response to weight on their backs are subtle–too slight to cause serious harm under normal circumstances. Clearly, horses the world over have been carrying riders for many centuries with little ill effect.

And yet, says Wickler, “we all also know that horses sometimes break limbs.” The California research lays a framework for understanding how adding weight to the horse increases the forces his limbs must withstand. As each foot strikes the ground, whatever force is not absorbed by bone and tendon must be taken up by the muscles. “If the muscle doesn’t produce the necessary force to handle the increase, the leg will collapse,” he adds, “so the horse must recruit more musculature.”

Fitness training increases and strengthens both muscle and bone, improving the horse’s reserve for absorbing the stresses of exertion, but at the extremes of equine athleticism cumulative stresses can be significant. “A small amount of weight can make a big difference,” Wickler says. “The addition of 10 percent of a horse’s weight may not be significant, but if he carries it over 100 miles, it might become important.” On the racetrack, the effects of a small amount of weight are magnified by the huge forces on the legs generated by galloping at extremely high speed. “For racing performance on a short track, 10 percent is a huge amount,” Wickler says. “Too much weight at too high a speed is asking for injury issues.”

But many pleasure horses carry heavier loads than sport horses ever do, sometimes for hours at a time, at various gaits over different terrain. While carrying a single heavy rider on a one-day ride is not likely to seriously harm a horse, over the years, a consistent regimen of this sort of work could add up to chronic injury.

The Cal State studies addressed muscular adaptations to carrying weight rather than orthopedics, and so they haven’t examined how weight might contribute to the occurrence of bone or joint problems. It’s possible that chronic overwork leads to many tiny microfractures, which can build up to a catastrophic break.

“It also makes sense that back pain might be associated with weight,” Wickler says. “There is a bit of normal extension and flexion during movement, and although the question has not been examined in detail, it’s likely that if you put a weight in the middle, you are going to change the way the back performs.”

How Much is Too Much? So how much weight can a horse safely carry? “While there seems to be some consensus, it isn’t as clear as one might think,” says Wickler. There is no definitive answer largely because there is no way to define the limits of safety. Obviously, a horse who staggers under a pack is overloaded. But that doesn’t mean that a horse who seems able to bear a heavy load is not accruing “silent” injury that will manifest years later as early arthritis or a sudden unexpected breakdown.

Time and terrain matter, too. The same horse who without apparent strain can handle a 250-pound rider in short sessions in the arena might be shaking with fatigue after an hour on a mountain trail.

In the absence of scientific research, the next source of information on maximum weight loads for horses comes from historical sources–the result of centuries of horsemanship experience, not all of which developed with the well-being of the horse as the highest priority. “U.S. Army specifications for pack mules state that ‘American mules can carry up to 20 percent of their body weight (150 to 300 pounds) for 15 to 20 miles per day in mountains,'” Wickler says. “There are some anecdotal reports of 350 to 400 pounds and even an 1867 reference to 600 to 800 pounds for mules.”

India’s Prevention of Cruelty to Draught and Pack Animals Rules, 1965, says the maximum for mules is 200 kilograms (about 440 pounds) and for ponies the maximum is 70 kilograms (154 pounds).

“Packers generally try to keep packs to 150 to 200 pounds in their animals, who must carry the dunnage on a daily basis for the entire season,” says Wickler, “so 20 percent of the animal’s body weight seems to be reasonable. However, these suggestions are for walking. If you go faster, that means more forces on the limbs and more metabolism is needed.” Today, many dude ranches and public stables post weight limits for riders, usually around 200 pounds or less; the National Park Service, for example, does not allow riders who weigh more than 200 pounds to participate in its mule trips into the Grand Canyon.

“The logical extension of this line of thinking is to never ride a horse or to make it a rule that only skinny people can ride,” says Wickler. “Obviously, that’s not going to happen. But people do need to be aware of the amount of weight they are putting on a horse.”

That includes not only the rider’s weight, but also the weight of the saddle, as well as everything else carried along. Western saddles engineered specifically for ranchwork or sports such as roping or cutting tend to be heavier, 40 pounds or more; those designed for trail or pleasure uses tend to be lighter, 25 to 30 pounds, but some models can range up to 40. Australian, endurance and synthetic Western saddles are lighter–with weights ranging from 13 to 22 pounds. English saddles vary somewhat by discipline but generally weigh 20 pounds or less, and some models weigh less than 10 pounds. Gel-filled saddle pads can add several pounds, as can any other gear worn by the rider or tucked into saddlebags.

The jury may still be out on exactly how all of this weight affects individual horses, but anything you can do to minimize the amount your horse carries will almost certainly benefit him over the long term. “I could stand to lose some weight,” says Wickler. “It’d be better for me, and it would also be better for my horse.”

This article originally appeared in the January 2005 issue of EQUUS magazine.