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It's easy enough to name the great horses: Secretariat, Seabiscuit, Affirmed, Alydar. Now name the men who rode them. When you've got almost 1,000 lb. of magnificent thoroughbred thundering along a track, it's hard to pay much mind to the little man riding on his back.

But we should. According to a new study just published in the journal Science, the greatest single increase in racehorse speed in the history of the sport occurred about a century ago and owed entirely to where the jockey did — or didn't — place his fanny on the saddle.

Until the end of the 19th century, all jockeys assumed a pose on the horse much like that of dressage riders: back straight, head up, seat planted firmly in place. The posture provided plenty of speed and plenty of control and, significantly, did not require the riders to support their own body weight — a real consideration over the course of a long race.

That's the way it was in the U.K., at least. In the U.S. — which always had a wilder, frontier relationship with its horses and merely borrowed the sport from the Brits anyway — the rules were looser. American jockeys of the time began wondering what would happen if they did a little work on their own, standing up in the stirrups, bending forward and surfing the motion of the horse as it galloped. What happened was, they went faster — 5% to 7% faster between 1890 and 1900, as more and more riders adopted the idea. That's a huge bump in speed in a sport that invented the term "win by a nose." In 1897, riders in the U.K. began picking up the practice, and by 1910, they were moving faster too.

The question was, How? Simply knowing that the pose works is not the same as knowing why it works — at least, not in the detail a physicist would like. Recently a group of researchers from the Royal Veterinary College in London decided to find out, using tools Edwardian sportsmen couldn't have imagined.

Thilo Pfau, a professor of bioengineering, and his team first outfitted both jockeys and horses with inertial sensors. The humans wore the instruments in their kidney belts; the animals wore them at the front of the saddle. "The sensors are accelerometers similar to what's in the Wii," says physicist Andrew Spence, who participated in the work. "Once you synchronize the two, you can determine the relative motion of the man and the horse." The jockeys also wore global-positioning trackers so their speed and position could be followed. "The tracker was in the helmet, where the GPS satellites could get a clear view of it," says Spence. The horses and riders were then sent running, and the biomechanical data poured in.

Any "jockey-plus-horse system," as the researchers call the racehorse-and-rider team, will start off essentially the same as any other: a combined mass of roughly 1,100 lb. (500 kg) of living flesh, with the horse representing about 87% of the total weight and the jockey making up the rest. One key to speed will be how lightly the horse can carry that 13% load. The investigators found that the horse's back oscillates up and down about 6 in. (150 mm) throughout its stride, and fore and aft about 4 in. (100 mm). The jockey moves too — up and down through a cycle of 2.3 in. (60 mm), and fore and aft just 0.8 in. (20 mm). That small motion makes a very big difference.

"Whether the jockey is sitting in the saddle or not, the horse still has to carry his weight," Spence says. "But by absorbing the jiggles of the horse, the jockey prevents the animal from having to make him go up and down with each stride. It's the difference between the horse carrying a moving rider or simply a quantity of lead that weighs the same." The crouched position the jockey assumes throughout pays an additional dividend by minimizing wind resistance.

In physics, however, nothing comes for free, and as the horse's workload goes down, the jockey's goes up. "The jockey's legs oscillate in length while transmitting a vertical force," the researchers wrote, "resulting in substantial mechanical work."

This minimize-the-load strategy operates in other places beyond the racetrack. We adopt it intuitively when we're carrying a heavy load and find ourselves maintaining an even, almost level gait, thus reducing the up and down motion that makes a bundle seem to weigh more on every downward bounce. Spence suggests that the same concept could be applied to improve the performance of artificial legs. "It's not crazy to say you'd be able to use this to build better prosthetic limbs for people. If you're trying to build a prosthetic limb to handle a bumpy environment, this should help," he says.

On the track, however, the ploy has most likely been thoroughly tapped. "My guess would be that the best jockeys already do everything they can in terms of using this mechanism," says Spence. "Where it could come into play in horse-racing is in training other people, to help them adjust their posture."

If strategy can't make the fastest horse-and-riders any faster, at least it promises more photo finishes.

• Jeffrey Kluger
• A new study on an age-old phenomenon explains why a jockey's minuscule movements can lead to great gains in a racehorse's speed