Kenyan athletes have dominated competitive middle- and long-distance running for more than half a century, a phenomenon suggested to be attributable, at least in part, to superior running economy. Given that lower-leg anthropometry is an important determinant of running economy, a key contributor to the athletic performance of Kenyan runners is thought to be slender lower legs. Running economy and lower-leg anthropometrics, including relative lower-leg length and mean lower-leg thickness, were measured in adult middle- to long-distance runners, including 12 Kenyan elite and 29 sub-elite runners, and 20 Danish elite and 37 sub-elite runners. Additionally, 46 untrained Kenyan and 30 untrained Danish adolescents were included. Among adult runners, the oxygen cost of running was 33.61 mL kg-0.75 km-1 lower in Kenyans compared with Danes [95% confidence interval (CI): 18.63, 48.59; p < 0.0001] and increased by 2.20 mL kg-0.75 km-1 per centimetre squared increase in lower-leg cross-sectional area (95% CI: 1.03, 3.37; p = 0.0005). In untrained adolescents, the oxygen cost of running was 44.82 mL kg-0.75 km-1 lower (95% CI: 21.99, 67.64; p = 0.0002) and increased by 1.75 mL kg-0.75 km-1 per centimetre squared increase in lower-leg cross-sectional area (95% CI: 0.18, 3.32; p = 0.0294). Slender lower legs are favourably associated with superior running economy and might contribute to the dominance of Kenyan athletes in competitive middle- to long-distance running. Highlights - What is the central question of this study? Is the superior running economy of Kenyans compared with non-Kenyans caused by more slender lower legs? - What is the main finding and its importance? Both adult Kenyan elite and sub-elite middle- to long-distance runners and untrained adolescents have superior running economy and slimmer lower legs compared with their Danish counterparts. Across countries and levels, more slender lower legs are closely associated with better running economy.
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