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

by Alex Hutchinson

There are many ways of delineating the boundary between short and uncomfortable high-intensity exercise and longer, more pleasant efforts. One of the most familiar is lactate threshold, the point at which you’re working hard enough that levels of lactate in your blood start creeping inexorably upward. A more recently developed concept is critical power, which is the point beyond which your muscles can no longer stay in the sustainable “steady state” equilibrium fetishized by Harvard Fatigue Laboratory researchers. Sixty minutes of all-out exercise, for a well-trained athlete, sits in the excruciating gap between these two markers, explains Mark Burnley, a physiologist in the University of Kent’s Endurance Research Group. “Riders in the Hour have to exercise above lactate threshold, but very slightly below the critical power—in other words, ride with the highest metabolic rate that is also steady state.” Done right, then, the Hour is literally the longest bout of painful high-intensity exercise you can endure.