Africa has a more diverse gene pool than any other continent plus a very diverse territory (mountains, high lands, rain forests, deserts, coastal areas etc).
Because of that diversity you are more likely to find individual people with extraordinary athletic talent there - be it reaction time or ability to run, put on muscle, have endurance etc., but that doesn't mean that the average African is more or less fit than any other people in the world.
Average fitness depends much more on lifestyle and culture than on race. Areas in the world where people walk a lot tend to have fitter average populations than areas where people drive even the smallest distances.
