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

by Alex Hutchinson

how much does the biggest force you can produce with a given muscle decline? Not surprisingly, he has found that the force produced by two key muscle groups in the legs, the quadriceps and the calves, gets progressively weaker as the distance of a running race increases—up to a point. By the time you’ve been out there for about 24 hours, your leg muscles will be 35 to 40 percent weaker, and they won’t lose much more. In fact, his Tor des Géants subjects, who took more than 100 hours, on average, to complete the race, ended up losing just 25 percent of their pre-race leg strength—a result that, on the surface, makes little sense. “Okay,” Millet jokes, “so if I run 200 miles, I’m less fatigued than if I run 100 miles!” This counterintuitive finding, on its own, already hints that leg muscles aren’t what ultimately limit ultra-endurance athletes. And there are some more subtle details. The electrically stimulated twitch data allows Millet to estimate how much of the force loss is due to fatigue in the muscles themselves, and how much is “central,” reflecting either diminished output from the brain or losses in transmission via the spinal cord. For ultra-endurance runs, it turns out, the muscles themselves typically only lose about 10 percent of their force-producing capacity; the rest is central, reflecting a progressive decline in the brain’s voluntary activation of muscle. “The brain is able to do more, but it doesn’t,” Millet says. But, he adds, that doesn’t necessarily me...