Athletes who dominate at sea level do not always look the same once a race climbs above 4,000 metres. That is one of the most fascinating realities in mountain running, and the Andes Mountain Skyrace is a perfect example.
In 2025, Anastasia started seven races and won six. But at the Andes Mountain Skyrace, one of the highest races on the calendar, the script changed. The course reached roughly 5,424 metres, making it a very different physiological challenge from a typical sea-level race.
Why altitude changes performance so much
The percentage of oxygen in the air stays about the same at altitude. What changes is barometric pressure. As pressure drops, there is less force pushing oxygen from the lungs into the blood, so every hard effort becomes more expensive.
That is why runners often feel as if they are breathing normally but still cannot produce the same power. The issue is not “less oxygen in the air” in a simple sense. The issue is that the body has a much harder time getting usable oxygen where it needs to go.
What happens to VO2 max at altitude
Research consistently shows that VO2 max begins to fall meaningfully above around 1,500–1,600 metres, and common estimates put the drop at roughly 6–11% per 1,000 metres, depending on the athlete and study design.
That means at a race topping out near 5,400 metres, even elite runners can be working with dramatically reduced aerobic capacity compared with sea level. In practical terms:
- the same pace costs far more
- lactate builds sooner
- fatigue arrives earlier
- technical mistakes become more likely late in the race
That is a huge deal in skyrunning, where pacing, footing and decision-making all get harder as oxygen availability drops.
Why native altitude athletes often look so comfortable
This is where athletes like Blanca Llumiquinga become especially interesting. At the 2025 Andes Mountain Skyrace, Blanca won the women’s race, while the event itself was widely framed around the brutal impact of cold and altitude.
Athletes who live and train at altitude often develop better tolerance to hypoxia and, in many cases, more efficient oxygen use during hard efforts. Long-term acclimatisation can reduce some of the normal performance drop seen when sea-level athletes first arrive at altitude.
So when the race reaches 4,000 metres and beyond, one athlete may be fighting a constant physiological crisis, while another is operating in conditions much closer to her norm.
That does not mean the sea-level athlete is less fit. It means the race environment is favouring a very specific adaptation.
It is not just fitness
There is also context. Travel, disrupted recovery, racing blocks and time away from home all matter. Performance is never shaped by one thing alone.
But at 5,400 metres, altitude is not just another variable. It can be the variable.
By the final descent, when precision and stability matter most, the athlete who is less physiologically stressed often has more left to use. That can be where the decisive move happens.
So how much altitude training would it take?
This is the big question.
The honest answer is: you can improve a lot, but matching someone who grew up at altitude is extremely difficult.
Studies suggest altitude camps and hypoxic training can improve adaptation, and some evidence shows interventions longer than three weeks tend to work better than shorter ones. But there is still a major difference between a training block and a lifetime of living high.
A sea-level runner may become far better prepared with:
- repeated altitude camps
- arriving early enough to acclimatise
- smart pacing adjustments
- specific strength and downhill conditioning
- realistic expectations around output
But fully reaching the same comfort level as someone raised around 3,000 metres is another matter. That advantage is not just about one camp or one month of prep. It is years of accumulated adaptation.
Final thought
Altitude training can absolutely narrow the gap.
It can improve tolerance, reduce the shock of hypoxia, and make an athlete more competitive when races climb high. But at the very highest elevations, there is still often a meaningful edge for athletes who grew up there.
So the real goal may not be to become identical to a native altitude athlete.
It may be to become good enough that altitude no longer decides the race before it begins.
