The purpose of the question is not to stir an asinine racial debate. It's because I am a former athlete of white descent, and I truly believe that African-Americans have more strength, speed, grace, and skill than whites and I am interested if there is proof of this, medically speaking and I'm not a chicken shit who is to afraid to open discussion on a topic that has been up for debate for a very long time.
Subjective and racist bullshit aside, I don't think there's a serious answer to your question. It may exist somewhere, but I haven't heard of any recent studies that proves or disproves this. In any case, I'm not sure that such a study would provide much usable information. It would be problematic to make inferences from it because qualities such as strength, speed, and skill are measurable, but potential is not. And those measurements are not fixed so they will change since no athlete will consistently deliver a 'perfect 10' performance every single time. Consider that world records are broken all the time, and by people of all different ethnicities. Ergo, the sprint runner who is the fastest and the best this week might get his ass beat by the a runner of a different ethnicity in next week's heats. Further, potential will vary not according to ethnicity but according to each individual and how much training, drive, and determination they have, and the activities within which the comparisons would be done.
Other factors that don't necessarily have anything to do with ethnicity can also affect performance. For instance, a very tall, thin, white person with long legs may be able to win a 100m race over a short-statured, heavier, more muscular African American person---not necessarily because the white person is actually a faster runner but because he's carrying less body weight and because his longer legs can cover twice as much distance as the African American person may be able to in a single stride.
Hitler and Mengele were notorious for conducting numerous experiments which they had hoped would prove their theory that one particular ethnic group is better than others in a variety ways---including intelligence, ability, and endurance. We all know the rest of that story. Since those days I don't know that any serious researcher would want to go down that road again, either because they already have enough empirical evidence to know that they would be wasting their time or because they know that it would be extremely politically incorrect to publish research data which suggests that perhaps the Nazis might have been on to something after all. That is a shitbox I'm sure no one would be in a hurry to jump into unless they want to become a pariah to the whole world.
