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.
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.
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:
"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."