At the conclusion of the men’s cross-country skiathlon in Sochi on Sunday, the competitors immediately collapsed in exhaustion. This always happens in cross-country skiing—the athletes invariably fall in a heap, looking like they couldn’t possibly move another inch. An NBC video titled “Finish Line Carnage” captures the phenomenon:

This sort of thing doesn’t happen in speed skating, or hockey, or ice dancing. Why?

Cross-country skiing is one of the most physically demanding sports known to man. Unlike their downhill colleagues, cross-country skiers don’t get much help from gravity. Unlike figure skating, cross-country skiing involves neither costumes nor choreography. Whereas running is primarily a lower-body activity, cross-country skiers use use all the major muscle groups to propel themselves across flat surfaces and inclines, with little opportunity for coasting, while the world yawns and changes the channel. You’d collapse, too—about 30 seconds after you started the race.

The post-race collapse is a common sight in lots of endurance sports. (Watch this video of a collegiate cross-country running event, starting around 8:30, for a glimpse of this, and also for a reminder of why you always need to remain alert if you’re standing near the finish line of a cross-country road race.) According to “Collapse in the Endurance Athlete,” a very helpful 2004 paper from the journal Sports Science Exchange, post-race collapses are usually not serious so long as the athlete “remains conscious with normal heart rate, blood pressure, breathing, and mental status.” (If an endurance athlete collapses during a race, that might be a sign of some more serious physical malady. That was the case with Torin Tucker, a 20-year-old Dartmouth student who collapsed and died during a cross-country ski event earlier this month; his death was later ascribed to an undiagnosed heart defect.)

Like all elite endurance athletes, great cross-country skiers are blessed with a high VO2max, or “maximal oxygen uptake,” which means their bodies are extraordinarily efficient at taking oxygen from the lungs and transporting it via the bloodstream to their muscles. It’s this high aerobic capacity that gives skiers the endurance necessary to complete a race.

And yet a high VO2max alone isn’t enough to win a race. Cross-country skiers must also be able to sprint. This sprint capacity is entirely anaerobic, which means the racer is summoning energy reserves stored in his muscles. These power bursts last for a short while, and they’re followed by a recovery period that usually involves heavy breathing, which floods the system with restorative oxygen. (Accessing anaerobic energy is sort of like flushing a toilet: All the water in the bowl surges down the drain, but then it’s gone, and you have to wait a little bit for the water to return to its previous level.)

Dr. Dan Heil, an exercise physiologist at Montana State University, suggested to me via email that it’s useful to think of Olympic cross-country skiers as some ungodly hybrid of wolves and cheetahs. “[M]odern cross country skiers need a high aerobic capacity to be able to hunt for a win (like a wolf relying upon aerobic capacity), but they also need a high anaerobic capacity to be able to sprint during the race, as well as for the win (like a cheetah sprinting for a kill),” writes Heil. “The consequence of utilizing both of these energy systems so completely for racing is a transient inability to stand upright on skis.”

Why? As a cross-country skier approaches the end of the race, he relies on these anaerobic bursts, with less and less time for recovery between each sprint. According to Dr. Heil, who has studied cross-country skiers, “repeated high intensity anaerobic efforts will cause a quick build-up of acid by-products in the muscles which causes the muscles to stop contracting effectively. This latter issue can be observed as a loss of coordination during the final sprint, as well as an inability to stand upright on narrow skis within the finishing chute.”

So you’re a cross-country ski racer approaching the finish line. As American Kikkan Randall puts it in the video at the top of this post, you’re “pushing yourself so close to those physical limits and you’re really just convincing yourself to last for incremental times longer.” You’ve been sprinting intermittently for the last couple of minutes, and your body is starved for oxygen. Your arms and legs feel like jelly thanks to acid buildup. “In short,” writes Dr. Heil, “you combine an insatiable need to breathe as hard as possible with leg and arm muscles seizing from acid build up, along with a central nervous system unable to control the act of standing and gliding upright, and the result is skiers dropping to the ground because they cannot stand and they need to recover.” And then it’s on to the next race, where they’ll work really hard and collapse yet again.

Read the rest of Slate’s coverage of the Sochi Olympics.