Small carbohydrate reserves can catastrophically limit human performance in endurance running: Runners risk "hitting the wall" if they exhaust their leg muscle glycogen before finishing a race. Using a mathematical model of carbohydrate metabolism during endurance running, Benjamin Rapoport (10.1371/journal.pcbi.1000960) demonstrates a method of computing personalized estimates of distances at which runners exhaust their carbohydrate stores while racing at various intensities, providing a quantitative basis for safe pacing strategies, optimal midrace fueling plans, and estimations of performance limits in human endurance running. This artistic rendering shows a runner at different distances and stages of glycogen depletion, superimposed on a glycogen molecule.

Image Credit: Benjamin I. Rapoport; segment of glycogen molecule from image by Mikael Häggström via Wikimedia Commons, brick wall courtesy of Charles S. Bond.