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Twenty-eight of the last 38 world record holders in the men's 100-meter dash have been black athletes, and researchers at two universities think they know why.

A new study by researchers at Howard University, a historically black school in Washington D.C., and Duke University in North Carolina suggests why black athletes may outperform athletes of other races in running events. Physical differences in the length of the limbs and the structure of the body mean the center of gravity tends to be higher in the bodies of black people, the researchers say.

Since 1968, the world record holders in the men's 100-meter dash have been black athletes. And since 1912, when the International Association of Athletics Federations started keeping track of the record holders in that event, only 10 non-black athletes out of 38 individuals have held the title.

"There is a whole body of evidence showing that there are distinct differences in body types among blacks and whites," said researcher Edward Jones, who researches adolescent obesity, nutrition and body composition at Howard University. "These are real patterns being described here. Whether the fastest sprinters are Jamaican, African or Canadian, most of them can be traced back generally to Western Africa."

Why center of gravity matters

Although there are also cultural factors at work, it all comes down to body makeup, Jones said.

"Blacks tend to have longer limbs with smaller circumferences, meaning that their centers of gravity are higher compared to whites of the same height," said Adrian Bejan, Jones' co-author, an engineering professor at Duke University. "Asians and whites tend to have longer torsos, so their centers of gravity are lower."

"These differences are small, and we don't really see them when we look at someone," Bejan told Life's Little Mysteries. "We are only rarely struck by how long someone's legs are."

But these small differences certainly matter in races lasting less than 10 seconds, Bejan said.

The height of a person's center of gravity affects how fast his feet are moving when they hit the ground, Bejan said. Each step a runner takes is like falling except the athlete breaks the fall with his foot. So the feet of a person with a higher center of gravity will hit the ground faster than someone with a lower center of gravity.

Torsos and legs

In the study, the scientists gathered data available from the militaries of 17 nations. Militaries measure their recruits for uniform fittings and are a reliable source of data, Bejan said. To approximate torso length, the scientists compared the average height of the military men with their sitting height – the distance from a chair to the top of the head.

Results showed the average sitting height of blacks was about 1.5 inches (3 cm) shorter than that of whites who were the same height. This means that, among blacks and whites of the same height, the legs of blacks were longer (think of a high-waisted person), while the torsos of whites were longer.

This physical difference gives a black athlete an advantage, even against an athlete of another race is who is taller and has a higher center of gravity, said Bejan. From a physics perspective, Bejan said, the legs do the work of running and the torso of the body is just extra weight that the legs must carry, so the race goes to the runners with longer legs and shorter torsos.

By contrast, whites tend to have the advantage in swimming, where a longer torso allows for faster speeds.

"Swimming actually generates a wave. The sport is the art of surfacing on that wave. When the wave is bigger – because the torso is longer – they go faster," Bejan said.

The study was published online this week in the International Journal of Design and Nature and Ecodynamics.