Once we crossed over the Italian border, Jornet steered the Peugeot through tight alpine streets to the small ski area of La Thuile. Ski-mountaineering racing, usually called SkiMo in Europe, is a mountain man’s steeplechase up, down and around high peaks while wearing ultralight backcountry gear that’s built to climb: matchstick skis, slipper-light carbon-fiber ski boots, climbing skins that grip the snow. The first race of the season was a few weeks away, and Jornet needed to log some workouts in the mountains. Beneath a giant trail map, we discussed a plan: he’d ski, and I’d try to watch. He wore a skintight cat suit adorned with tiger stripes the color of traffic cones. KILIAN was printed across his right thigh. The message was less boast than warning: Get off the tracks. Train coming.

At one point during Jornet’s workout that day — he’d climb and then descend about 10,000 feet in about four hours — he paused at a mountaintop cafe to talk. I offered espresso. He declined. He also hadn’t eaten breakfast, nor would he eat or drink during his workout.

Don’t you sweat? I asked.

“Maybe a bit here,” he replied, touching the back of his neck.

Even among top athletes, Jornet is an outlier. Take his VO2 max, a measure of a person’s ability to consume oxygen and a factor in determining aerobic endurance. An average male’s VO2 max is 45 to 55 ml/kg/min. A college-level 10,000-meter runner’s max is typically 60 to 70. Jornet’s VO2 max is 89.5 — one of the highest recorded, according to Daniel Brotons Cuixart, a sports specialist at the University of Barcelona who tested Jornet last fall. Jornet simply has more men in the engine room, shoveling coal. “I’ve not seen any athletes higher than the low 80s, and we’ve tested some elite athletes,” says Edward Coyle, director of the Human Performance Laboratory at the University of Texas at Austin, who has studied the limits of human exercise performance for three decades.

Born into a Catalan family, Jornet grew up in the Spanish Pyrenees at 6,500 feet, and his gifts are literally in his blood. “When you are born and bred at altitude, you tend to have a higher blood volume and red-cell count for oxygen-carrying capacity,” which translates to better endurance, says Stacy Sims, a researcher at Stanford who holds a doctorate in exercise physiology and nutrition science. Years of daily running and skiing up mountains have further bolstered this advantage. This helps explain why Jornet sweats so little. During exercise, the bodies of very fit people quickly act to disperse heat by, among other things, vasodilation — expanding blood vessels at the skin’s surface where the air can cool the body. A body that sweats less loses less precious liquid from its circulatory system, a major factor in fatigue. In moderate temperatures, Jornet says, he can run easily for eight hours without drinking water.

Jornet was raised in the Cap del Rec regional park, where his father was a hut keeper and mountain guide and his mother a schoolteacher who liked to run and ski. “Mountains were his playground,” his mother, Núria Burgada Burón, told me. When Jornet was 18 months old, she took him on a seven-hour hike in the Pyrenees, and he never cried or fussed. Seven hours? She laughed. “Kilian is not normal.” At 3, she says, he completed a 7.5-mile cross-country ski race. “My mission is to make Kilian tired. Always, I was tired. But Kilian? No.”

His parents tried to instill a sense of humility and a deep feeling for the landscape. “Por las noches we walk out to the wood, the forest, without lamp,” Burgada says, describing how she sometimes took Jornet and his sister, Naila, a year and a half younger (and today also a SkiMo racer), out barefoot into the night dressed only in pajamas. Listen to the forest, their mother told them. Feel the direction of the wind against your cheeks, the way the pebbles change underfoot. Then she made her children lead the way home in the darkness. “All this,” she says, “to feel the passion of the nature.” At 13 Jornet entered a program for young Catalan ski-mountaineering athletes; he won his first youth World Cup race at 16. He began to run as off-season training.