I've often heard it argued that the reason why African Americans are overrepresented in the sporting field in the USA is because they are descended from a stock of slaves that for hundreds of years were selectively bred. And that slave-descended Africans in the New World and beyond are generally fitter and stronger than the Africans who live now, and always have, lived in Africa (though no doubt, diet and healthcare play a role in this).
That's absolutely ridiculous. Most blacks in America have ancestors who come from Western Africa. The reason that African-Americans do well in sports is because West Africans on average have a higher percentage of fast-twitch muscle fibers allowing them to be good sprinters etc.
On the other hand, East Africans, like Kenyans, have on average a higher percentage of slow-twitch muscles allowing them to be good long distance runners. This is why American blacks tend to do well in short events, but also explains why there are not very many successful American black marathoners.
It has nothing to do with eugenics, and everything to do with the different phenotypes of populations living in two different areas.
Lastly, blacks are only over-represented in a few sports such as football and basketball. Most sports have a dearth of black faces. Hockey, Swimming, and Tennis come to mind.