But it’s less clear where that fatigue originates. It’s pretty obvious that your quadriceps and calves will be weaker right after you run 100 miles. The range of distances provided a perfect opportunity to test how leg muscle fatigue responds to different durations of running. The men’s winning time in 2019 was over 20 hours. The distance is just part of the story: the longest race, for example, also includes almost 33,000 feet of climbing and descending. The runners studied in 2019 ran one of two “short” distances (25 and 34 miles) or three “long” ones (62, 90, and 105 miles). This year, about 10,000 runners will participate in seven different UTMB races over the course of a week in late August. So Millet and his colleagues recently published the sixth in a series of studies from that 2019 UTMB. Here are some of the insights from those studies about what it takes to run through the mountains for hours on end, and how your body responds to the challenge. But you get more plentiful and realistic data by studying ultrarunners in the wild. Can you really get volunteers to run on a laboratory treadmill for 24 hours? Well, yes, you can-and Guillaume Millet, a researcher at the Université Jean Monnet Saint-Etienne in France (and himself an accomplished ultrarunner) has been there, done that, and published the paper. Running ultras is hard so is studying them. Many of the super-fit ultrarunners couldn’t stay on the treadmill for that long, so the scientists altered the protocol on the fly and shortened the treadmill run to three minutes. The problem: one of the tests required them to run on a treadmill for four minutes while their breathing was measured.

Waiting there was a team of scientists with a roomful of lab equipment to measure the precise physiological toll of their exertions.

Immediately after stumbling across the finish line of the 2019 Ultra-Trail du Mont-Blanc, the legendarily punishing mountain-ultra-trail event, 56 runners hobbled over to the National Ski and Mountaineering School in the French resort town of Chamonix.

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