Altitude exposure typically refers to elevations of 1,800–2,000 m above sea level (a.s.l.) or higher. However, elevations of 1,000–1,500 m a.s.l. are often already classified as mild altitude.
The defining feature of altitude is the reduced partial pressure of oxygen. In simple terms, the percentage of oxygen in the air remains the same—about 21%—but the air pressure is lower. As a result, every breath contains fewer oxygen molecules available to enter your bloodstream (hypobaric, hypoxia). As you can intuitively understand, the lower partial pressure of oxygen decrease aerobic performance. How? Following the PMEP’s Cardiovascular-Anaerobic Model of Endurance Performance, it makes less oxygen moves from your lungs into your blood, making it harder for your muscles to receive the oxygen they need to produce energy aerobically. As a result, VO2max and power at thresholds are generally reduce. This is why endurance exercise feels more difficult at altitude.
An interesting question is:
At what altitude does performance start to decline? Is there a specific threshold below which altitude has little or no effect, or does performance decrease continuously as altitude increases? And if it does decrease continuously, is the relationship linear or does the decline become progressively steeper with increasing altitude?
A study published by Wehrlin and Hallén (Norwegian School of Sport Sciences, Oslo, Norway) on European Journal of Applied Physiology in 2006 tried to answer this question. (1)
WHAT DID THEY DO?
8 endurance trained runners were recruited: age 24, VO2max 61–74 ml/min/kg.
On separate days and in a randomized order, all participants completed a time-to-exhaustion running test at 107% of the speed associated with VO₂max (107% vVO2max), which had been determined at “sea level” (300 m a.s.l.).
The tests were performed at the following altitudes:
300 m a.s.l.
800 m a.s.l.
1300 m a.s.l.
1800 m a.s.l.
2300 m a.s.l.
2500 m a.s.l.
The different altitudes were simulated in a hypobaric chamber, and all tests were performed on a motorized treadmill.
The aim was to examine how endurance performance and VO₂max change across the different altitudes. Performance was assessed as time to exhaustion, while VO₂max was determined as the highest oxygen uptake reached during the time-to-exhaustion test performed at 107% of vVO₂max.
