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."
That in itself should qualify a winning jockey as a champion athlete as well as the horse he rode in on. Link -Thanks, Alyson!
(image credit: Eadweard Muybridge)