The genes
According to scientists, the Kalejins have genes that make them the best runners in the world. The European studies indicated that the Kalenjin have less mass for their height, longer legs, shorter torsos, and more slender limbs. These features have been described as “bird-like,” which indicate that they make them more efficient runners, especially over long distances.
According to another researcher, David Epstein
“Thin ankles and calves, a body build common to Nilotic tribes who grow up near the equator is particularly important in running because your leg is like a pendulum. The more weight you have farther away from your center of gravity, the more difficult it is to swing.”