Scientists have found the reason why blacks dominate on the running track and whites in the swimming pool - it's in their belly-buttons.

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.

Nary a button in sight at the Olympics swimming. Credit:Vince Caligiuri

The navel is the centre 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 centre of gravity," Duke University professor Andre Bejan said.

"It so happens that in the architecture of the human body of West African-origin runners, the centre of gravity is significantly higher than in runners of European origin", which puts them at an advantage in sprints on the track, he said.