Krellin, buddy, you need to settle down, especially since you're mistaken and I know that can be hard to hear that, particularly when one is in a hostile mood.
Here's a brief list of what's wrong with your conception of American slavery and genetics:
1. Slave masters can hardly be said to have bred for any given set of abilities, much less engage in behavior that might rise to the term 'selectively breed.' Our pre-Mendelian understanding of genetics was uneven at best. If (and this is a big if) a slaveholder was trying to engage in conscious, scientific breeding of slaves, he would have as likely been looking at phrenological signs or the color of stool samples or how much bile a person produced as he would have been looking for things we today consider genetic markers.
2. Slave masters generally didn't intentionally breed slaves for characteristics like strength and endurance. I have never seen any reliable evidence that it was a widespread practice for a considerable length of time on a single plantation, much less a city or region, much less the entire South. Moreover, slaves themselves chose sexual partners (outside of white-owner-on-black-slave rape or similar circumstances) the vast majority of the time.
3. There was not enough fluidity in the slave market to make physical characteristics a defining adaptive advantage, particularly important for this discussion because physical characteristics are not themselves 100% indicators for athletic skill (Matt Stafford looks like he should be hanging drywall). Most slaveholders got slaves from their existing slave population's natural reproduction, from family or kin networks, and from the small selection of a local market. There was no slave E-Bay.
4. Psychological characteristics were far more important in slave success than physical ones. The difference between the strongest cotton picker and the weakest cotton picker is less important, from a slavery perspective, than between the most obedient slave and the least obedient.
5. Related: The kind of physical traits a life of slavery favored are not the same ones that make you a good track star or football player. Physical endurance in cotton picking season is not even in the same ball park as physical endurance in a three-hour athletic event. Plus, far more important than "athletic" physical abilities would be "lifestyle" physical ones, like being able to work without sufficient nutrition or avoid illness without good clothing.
6. This doesn't explain the recent relative success in certain athletic endeavors of Africans who never came to the Americas, nor of the successes of other racial groups in other athletic events (I don't think Michael Phelps' ancestors were selectively bred for swimming).
7. There wasn't enough time for this to have any effect even if it existed. Fruit flies are not a good comparison because they live eight days. Even if we go all the way back to the first slaves brought to the USA, that's barely a dozen generations. That's enough time for small changes if an expert breeder is raising dogs, but not nearly enough time for varied and unscientific slave masters to meaningfully change the physical features of their slaves.
8. There are better explanations for African and African-American athletic success than this silly pseudo-science.
9. Most importantly: There was a lot of intermarriage, random sexual encounters, and rape between white and black southerners, plus a constant influx of new black (before the end of the slave trade) and white people into the breeding population. There's barely a "black" gene pool to even be having a conversation about.