“Imagine running in soft sand, and how hard that is. Now add upper body muscle demands to that, for 10-15 kilometers at 20-25 kilometers per hour,” said Stacy Sims, an exercise physiologist. “The cardiac output demand is super high.

“It’s often called ‘the ultimate endurance sport.”

Sims was talking about cross-country ski racing, and she’s not alone in her evaluation of that sport as savagely demanding.

“Elite XC ski racing is essentially non-stop intervals which, of course, is highly reliant on both anaerobic energy (dominant during the intervals) and aerobic energy (dominant during the recoveries) to be successful,” said Dan Heil, an exercise physiologist at Montana State University.

“There is certainly no other endurance sport that equals elite XC ski racing’s high reliance on both of these systems.”

Because cross-country skiing requires participants to use their ski poles for propulsion - as opposed to downhill racing, where they’re used for balance - the upper body is worked as hard as the legs, providing a uniquely challenging work-out.

This is backed up by scientific research from Sweden and Indiana, where scientists assembled groups of octogenarian men who were healthy enough to complete vigorous exercise tests to exhaustion.

One group consisted of lifelong cross-country skiers who trained four to six times a week, while the others didn't do any formal exercise beyond the activities of daily living.

It’s hardly a surprise that the skiers were fitter than the non-skiers, but the Journal of Applied Physiology published findings which showed the skiers had approximately twice the cardiovascular and muscular fitness of the untrained group. When compared to the results of previous studies involving lifelong endurance athletes who had also exercised into their eighties, the skiers were still far fitter, by about 40 per cent.

The scientists concluded that the skiers’ fitness "places them in the lowest all-cause mortality risk category for men of any age.”

Another long-running analysis of skiers, one involving 73,000 men and women over ten years in an annual long-distance race series in Sweden, found that they were less than half as likely to die during the follow-up period as similar groups of people from the general population.

One of the reasons which makes cross-country skiing so much better than other forms of endurance exercise, like running and cycling in particular, is that your upper body plays a big role. US skier Jessie Diggins made this point to Business Insider before the Pyeongchang Games: "People used to see us [as] these little forest fairies in spandex, and we go up into the woods and come back two hours later and that was the race. It's so much more dynamic than people realise.

“Here comes Diggins! Yes! Yes! Yes!” This Tony Romo-sound-alike announcer just got me out of my seat...for cross country skiing. #usa #OlympicGames2018 pic.twitter.com/nwh8AvgzGr

— Brad Howe (@bradhowe07) February 21, 2018

“It takes everything you have to get around that course, because you're working your legs, your arms, your core," she added. "The way I'm describing it, it sounds like it couldn't possibly be fun because it's so hard. But it actually is fun.”

By the way, the two experts quoted above - Heil and Sims - were speaking to Bill Bradley for a piece on deadspin.com which centred on Bradley’s experiences as a cross-country skier. Those experiences revolved around vomiting at the end of races, which is as common as collapsing at the finishing line, something cross-country skiing is famous for.

Which more or less bears out what we were saying.

If you’re interested in learning more look up snowsports.ie