Blacks (African Americans to be specific) generally have more fast-twitch muscles than other races. This makes them better at sprinting. Whites (Euro-Americans) have more slow-twitch muscles. This makes them better at long-distance races. In my experience, black dudes can get 'cut' or 'ripped' easier, while white dudes can build more muscle, and get bigger, though not as ripped. Their are always many exceptions to the rule, of course.
Living in America and being a huge basketball and football fan, it is difficult to think that blacks aren't better than whites at sports, or at least the sports I care about. When one race makes up 10% of the total population, and 90% or more (percentages are guesswork) of the NFL and NBA, I would pretty much say that the race has more natural athletes than the other.
My theory, based on nothing scientific, is something like this: during the slave trade, Africans were bought, sold, and traded for from West Africa. The slave owners that ran big plantations (a very large percentage, if not most of the slaves ended up in places like this) did not want their slaves to be able to communicate and conspire against them. They probably mixed slave groups so that there wasn't a big group that could talk and figure out how to escape or kill the slave owners, etc. This was possible because many of the tribes spoke completely different languages.
So you have huge groups of slaves, and everyone comes from a very different part of West Africa. The gene pool is large and varied, and black American slave children often inherited very good genes. This resulted in what we might today call natural athleticism and natural intelligence (though the slave owners did whatever they could to kill the intelligence, I could say more on this but I'll spare y'all).
Also, the slave owners often had children with the slaves, making the gene pool even larger and even more varied.
I have this natural talent theory because I beleive that (specific to this case, football and basketball) athletes of all races try their hardest to get to the pro's, if they are exceptional at the lower levels. The percentages that try their hardest are the same. The "x-factor" is the natural ability. I believe that it is not racist to think that African Americans have more natural ability in certain sports, especially given the environment I grew up in. Basketball and football require very advanced coordination as well, which could say something about some of the brain wiring of African Americans, that it is on average more advanced than other races. It is just my opinion that basketball and football players are the 'best' athletes, and that their sports require more athleticism than any other sports. So my theory is based on a lot of opinion.
Then again, I was the only white kid on a black football team of about 40 kids, and was the fastest kid on the team. Their are always exceptions to the rule
Edit: My theory is essentially my opinion, I am not presenting anything as fact. Please feel free to correct me if my history is wrong, just don't be mean about it!