The Science of Altitude
So, how does training on top of a mountain impact your fitness? It all has to do with how your body adapts to changes in atmospheric pressure. To understand more, there are two things you should know about barometric pressure:
Okay...but what does any of this have to do with fitness and altitude training? Well, let's combine what we learnt about pressure and apply it to high altitude:
We know that the atmospheric pressure is lower up there. And we can also imagine that breathing would be more difficult, since the body would have to work harder to push air into the lungs, and therefore the bloodstream (without the help the larger pressure gradient that breathing at sea level allows).
So how does the body rise to the challenge?
When given time, our bodies are great at adapting in these kinds of environments. In order to accommodate to the lower atmospheric pressure, the body produces more red blood cells. Boosted red blood cell production translates to lots of fitness-related benefits, like improved VO2 max (oxygen consumption) and lactate threshold (the max effort you can hold until that burn sets in).
This explains why up to 80% of Olympic athletes head to training camps in places like Flagstaff, AZ and Colorado as part of their training.