Cross country skiing has been shown to be one of the best forms of exercise available. It uses all muscle groups in the body to produce movement. The legs are primarily used for locomotion, but the arms make a large contribution to the power developed. Even muscles that at first sight do not appear to be involved are actively used in maintaining balance and core stability. Also because the activity is spread across many muscle groups, without any one being stressed, activity can be maintained for long periods. This leads to improvement in cardiovascular function and the ability to deal with the lactic acid build-up. The sport has huge calorie burning potential, from 600 calories/hr for recreational skiers, to just over 1000 calories/hr for the top athletes. Indeed, many research papers have shown that this full body workout has resulted in extremely high VO₂ Max figures.  Cross country skiers have been recorded as having the highest values of any sportsmen or women.

External links to detailed articles:

http://www.healthfitnessrevolution.com/top-10-health-benefits-cross-country-skiing/

https://www.theglobeandmail.com/life/health-and-fitness/fitness/the-jaw-dropping-benefits-of-cross-country-skiing/article6747824/

https://www.chatelaine.com/health/fitness/health-benefits-of-cross-country-skiing/

http://www.humankinetics.com/excerpts/excerpts/learn-the-benefits-of-cross-country-skiing

https://www.fatsecret.com/fitness/skiing-(cross-country)

http://crosscountryskicolorado.com/fitness/fitness.html

https://en.wikipedia.org/wiki/VO2_max

https://www.topendsports.com/testing/records/vo2max.htm

https://fasterskier.com/fsarticle/hofstad-closing-in-on-the-vo2-max-record/