What's important is not whether an athlete has an innie or an outie but where his or her navel is in relation to the rest of the body, says the study published in the International Journal of Design and Nature and Ecodynamics.
The navel is the center of gravity of the body, and given two runners or swimmers of the same height, one black and one white, "what matters is not total height but the position of the belly-button, or center of gravity," Duke University professor Andre Bejan, the lead author of the study, told AFP.
"It so happens that in the architecture of the human body of West African-origin runners, the center of gravity is significantly higher than in runners of European origin," which puts them at an advantage in sprints on the track, he said.
Individuals of West African-origin have longer legs than European-origin athletes, which means their belly-buttons are three centimeters (1.18 inches) higher than whites', said Bejan.
That means the black athletes have a "hidden height" that is three percent greater than whites', which gives them a significant speed advantage on the track.
"Locomotion is essentially a continual process of falling forward, and mass that falls from a higher altitude, falls faster," Bejan explained.
In the pool, meanwhile, whites have the advantage because they have longer torsos, making their belly-buttons lower in the general scheme of body architecture.
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