Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance

by Alex Hutchinson

In 1935, an international team of scientists led by David Bruce Dill of the Harvard Fatigue Lab ventured to Chile, where they outfitted a mobile laboratory in a train car and journeyed from sea level to a sulfur mine on the upper slopes of a 20,000-foot-high volcano called Aucanquilcha, putting themselves and other volunteers through exhaustive experiments at various elevations along the route. In the process, they identified a puzzling and still-controversial phenomenon known as the “lactate paradox.”43 Under ordinary circumstances, you produce high levels of lactate in your muscles and blood when you “go anaerobic”—that is, when you’re exercising so hard that your muscles can’t get fuel quickly enough from the usual oxygen-dependent aerobic pathways. As you go to higher altitudes, with less oxygen in the air, you would expect to go anaerobic sooner, and produce more lactate at a given pace or power output. Instead, Dill’s team observed the opposite: the higher they went, the lower the lactate levels they were able to produce at exhaustion. Extrapolating from their data (which has since been reproduced and reconfirmed many times) suggests that by the time you reach 23,000 feet, where oxygen levels are less than half their sea-level values, you won’t be able to raise your lactate levels at all. A series of studies by Markus Amann and his colleagues, involving 5K cycling time trials and rides to exhaustion at a range of simulated altitudes, offers a possible explanation for...