I don't really like to get into race discussions but I was reading a study on Olympic athletes and they published how athletes at certain sports disciplines tend to have all similar physical skeletal indexes and body compositions that give them mechanical advantage at what they do (ratios of limbs and other skeletal measurements to one another) and that it is no coincidence that your average joe on the streets can't just train and become an olympic sprinter or wrestler or w/e. The people who master athletics like that have certain physical gifts that allowed them to specialize at what they do.
In that study race was pointed out as a sidenote observation. When they took measurements of everyone, they typically (and remember, there are ALWAYS exceptions to rules) but typically your average athlete with recent African ancestry had a shorter skelic index (a smaller torso with longer limbs), relatively wider shoulders, relatively narrower pelvis, and a relatively higher crural index (a relatively longer tibya/fibya in relation to the femur) than peoples of recent European ancestry and the same differences get exaggerated a bit further still from peoples of recent East Asian ancestry. The traits described offer mechanical advantage in running and leaping activities. Again, this is not to make a sweeping generalization because everyone's body is totally unique to themselves and you can be white or asian and have long legs and broad shoulders just like you can be black and have the opposite, but the frequency of genes that contribute to let's say all the ingredients that have to go into making an idealized "runners" body seems to be more common in people's who come from populations that were of mostly recently African origin.
To anyone who studies biology this actually should not be a surprise. In nature, populations of animals that live closer to equalateral regions generally have longer limb proportions than their identical species counterparts that live in cooler regions. Natural selection is in play, any marginal increase running efficiency of longer-limbed animals isn't the driving force either, that's just a byproduct of what's going on. What's really happening in nature is long limbed small-bodied animals over time suffer more in cooler climates, and shorter limbed larger bodied animals suffer more in warmer climates. It's harder to keep your hands from getting frost bite in the Swiss Alps if you have a 6-6 wingspan on your 5'11" slender frame with a smaller waist/torso (your furnace) supplying warm blood to those limbs. Inversely, if you have a stout 5-11 frame where a greater deal of that height comes from a longer thicker torso with slightly shorter limbs (let's say a 5-11 wingspan) than your going to be at higher risk of heat stroke trying to keep your body temperature down at mid noon in the dry season on a scorching 112* day in the African Savannah. Think back to the most recent ice age where most of the worlds populations diverged and became temporarily isolated from one another under very harsh climatic conditions and it helps explain some of the subtle physical differences among what we today call "races" of humans. Race is an ugly blanket statement, because in the recent years geneticists have discovered that we actually all have the exact same genes present in every population, and there is not one gene exclusive to Africans nor is there one gene exclusive to White people, nor Asians etc, the only thing that seperates us is gene "frequency" in populations (for example European people are more likely to have a mutation to an Mc1r1 gene that affects melanin and hair color (a redhead related gene I believe)... but really anyone can potentially have the mutation all accross the globe). The differences are for the most part just superficial. Though, a bit of the climatic adaptations may be spilling over into our modern world of athletics and contributing to the lack of "white guys" performing specialized roles like NFL running back. There may well be a few white guys who are properly proportioned to be a great running back, but it may just be rarer for a person with european ancestry to be born with the best bodyplan to do it. That's my take on it at least. It could just be psychological too like Mastermind pointed out, but I can see it being a little bit of both actually.
