I am not trying to sound racist, I wonder how the selective breeding of southern slave owners affect modern blacks?
Besides his dedication and nutrition, genetics has to play a role.
It is something society does not like to talk about, but it happened.
Probably very little. Humans across all races are very similar if you take into account proper nutritional levels and everything. Remember that every black male that survived, he was likely sold for some type of a purpose. And that purchasor would have bred whoever he had, if he had that opportunity.
Back then, in a life of hard labor, being huge and built would work against you because you needed more calories to survive.
Also, white men had a large part in breeding as well, with their own DNA.
Look at Asian men, they are now very close to the same average height as men in the USA. You don't go to Japan anymore and tower over everyone.
One big issue here is perception. When a white man sees a well built black man, they tend to get that slave ownership perception that's been ingrained in our heads and look at him like a piece of cattle. That doesn't happen so often when you look at well built white men.
On average, white men and black men are practically the same height. There isn't any difference, really, other then that of perception.
Also, social differences are what causes the other perceptional changes. I'd venture an uneducated guess and say that basketball is a much more common sport to play as a child in a large urban city vs. other sports. Since black people overwhelmingly live in urban centers, that means black people disproportionately are involved in playing basketball.
Because of the sheer numbers of those playing, you will have a better chance of finding more adept black basketball players then white players.
Who knows, that white guy playing football, soccer, or golf, he might have been a natural basketball player. But you would never know, because it's just not a hugely popular sport for white people to play.