The reduced oxygen concentration prompts the generation of new red blood cells. That translates to a higher VO2-max back at regular altitudes.

High altitude has been well known as a training aid for decades, e.g. Kenyan marathoners: https://www.cnn.com/2019/11/06/africa/kenya-runners-win-mara...

The US Olympic Training Center is in Colorado Springs, Colorado.

Elevation: 6k ft and next to Peaks Pike at 14k feet.

I used to go there in the late 80's as a cyclist to train.

They'd follow you in a car as you rode up PP, honking at you to go faster.

But if that's what they wanted to replicate they could use nitrogen, or carbon dioxide, or something else that isn't actively poisonous.

Carbon dioxide is actively poisonous.

But anyway, that requires doing the exercise inside the controlled atmosphere. I'd guess the lure of carbon monoxide is that you can breath it, go out and exercise on a normal atmosphere. (That is, if you don't die on the first step.)

But then, if I had to guess I'd say the fact that the effects are long-term would reduce the athlete's performance. So yeah, I'd guess wrong.

You're right, carbon dioxide in large enough doses is no bueno, but even then it's more likely an issue of asphyxiation than anything else, and it's really a matter of scale. Carbon dioxide exist at ~400 ppm and safe at 5,000 ppm over 8 hours, compared to 50 ppm for carbon monoxide[1]. I know which one I'd rather use to substitute some oxygen for.

I was just trying to point out that if they wanted to induce hypoxia, there's better gases to be inhaling.

[1] https://www.indsci.com/en/blog/carbon-monoxide-vs.-carbon-di...

They probably need something that has longer negative effects. They need something that is slightly poisonous.

They probably need something that has longer negative effects. They need something that is slightly poisonous.

You'll also see a lot of MMA fighters train up in the mountains to the same extent.

It's quite common, and how it's done is often depending on where you compete (if you need high altitude experience during the race itself). So some practice sleep high train low, while others are high and do the workouts as well at altitude.

Many athletes also sleep in altitute tents. Basically something you wrap around your bed or a mask, that simulates being at altitute.