A team of German researchers explored this question by comparing VO2max, certain physiological determinants of VO2max, and running economy in Kenyan and German elite runners. They found no differences in total hemoglobin mass or blood volume between the two groups, while heart volume was slightly lower in the Kenyans. VO2max was also the same in Kenyans and Germans.
A treadmill test of running economy revealed that the Kenyan runners were more economical at speeds exceeding 18 kph (11.16 mph). This finding confirms previous research. One past study in particular isolated the smaller calf mass of Kenyan runners as a likely source of their better “gas mileage”. Weight below the knee contributes to the total energy cost of running more than weight anywhere else in the body.
Speaking of weight ,the biggest difference between Kenyan and German runners observed in the new study was that the former were indeed quite a bit smaller-nearly 18 pounds smaller. Could it be that simple?