Anyone who has tried running, cycling, or simply climbing a mountain at high altitude has probably noticed something strange: the same effort that feels comfortable at sea level can become much harder several thousand meters above it.
Breathing feels more difficult, heart rate increases, and pace needs to be reduced. But why does this happen?
The main explanation is the lower availability of oxygen, which changes the way the body transports and uses oxygen during exercise.
The Air Does Not Have Less Oxygen, But the Pressure Is Lower
An important point is that the percentage of oxygen in the air remains close to 21% even at high altitudes. What changes is atmospheric pressure.
As altitude increases, barometric pressure decreases. As a result, the partial pressure of oxygen in the air we breathe also decreases. This means there is less pressure driving oxygen from the lungs into the bloodstream.
This phenomenon is known as hypoxia, a state in which less oxygen is available to the body's tissues.
During exercise, this becomes even more important because the muscles' energy demands increase precisely when oxygen availability is reduced.
What Happens to VO₂max?
VO₂max represents the body's maximum capacity to take in, transport, and use oxygen during exercise.
At altitude, this capacity decreases. Because less oxygen is available to be transported through the blood, the body has more difficulty maintaining the maximum oxygen consumption that can be achieved at sea level.
In practice, this means that an intensity corresponding to a certain percentage of VO₂max at sea level may represent a much higher relative intensity at altitude.
That is why an athlete may need to slow down even while maintaining a similar perceived level of effort.
The reduction in aerobic capacity tends to become greater as altitude increases.
The Heart and Lungs Have to Work Harder
The body tries to compensate for the lower availability of oxygen by increasing ventilation and modifying its cardiovascular response.
Breathing becomes faster and deeper to increase oxygen intake into the lungs. At the same time, heart rate tends to increase at the same exercise intensity.
In other words, the body has to work harder to perform the same amount of work.
Increased ventilation also raises the workload of the respiratory muscles, while cardiovascular adjustments help maintain oxygen delivery to the tissues.
Why Does Pace Drop So Much in Endurance Sports?
Running, cycling, triathlon, and other endurance sports rely heavily on aerobic metabolism.
When oxygen availability decreases, the ability to sustain high intensities is also affected. This can result in lower running speed, reduced power output, or greater difficulty maintaining a given pace over longer periods.
In sports that require repeated high-intensity efforts, such as soccer, hypoxia can also impair the ability to perform successive accelerations and maintain performance throughout the activity.
And there is another important detail: not every athlete responds to altitude in the same way.
Endurance athletes who experience greater reductions in oxygen saturation during exercise may experience a more pronounced decline in performance when exposed to altitude.
The Body Can Adapt
The good news is that the body does not simply accept the lower availability of oxygen.
With exposure to altitude, several physiological adaptations begin to occur.
One of the best-known mechanisms involves increased production of erythropoietin, or EPO, a hormone produced mainly by the kidneys that stimulates red blood cell production.
Over time, changes in total hemoglobin mass can improve the body's oxygen-carrying capacity. This is one of the reasons altitude training strategies are used by endurance athletes.
However, adaptation does not mean that an athlete will immediately return to the same performance level seen at sea level. The process depends on altitude, duration of exposure, individual characteristics, and training strategy.
Is Training at Altitude Always Better?
Not necessarily.
There is an important difference between competing at altitude and using altitude as a training strategy.
Acute exposure to altitude tends to impair performance. On the other hand, some hypoxic training strategies can stimulate interesting physiological adaptations.
One of the most studied approaches is the Live High, Train Low model, in which athletes live at altitude but perform part of their training at lower elevations. The goal is to take advantage of some of the physiological effects of hypoxia without compromising the ability to perform high-intensity training as much.
Research has shown that this strategy can promote adaptations related to oxygen transport and, in several studies, improve VO₂max, time-trial performance, and maximal power.
Still, the results are not consistent across all athletes. Different protocols, altitudes, exposure durations, and individual characteristics can produce different responses. Therefore, altitude should not be considered a universal strategy for improving performance.
What About Hydration?
Hydration also deserves attention at altitude.
Exposure to altitude can affect fluid balance and increase water loss, particularly because of increased ventilation. During prolonged exercise, this can make hydration strategy even more important.
In addition, when exercise takes place above approximately 2,500 meters, environmental and physiological factors associated with altitude can make hydration more complex than at sea level.
This does not mean that athletes should simply drink as much as possible. Hydration strategies should consider exercise duration and intensity, environmental conditions, sweat rate, and individual tolerance.
Altitude Changes Perceived Effort
Perhaps one of the most noticeable changes is the feeling of effort itself.
An athlete may look at their watch and realize that they are running considerably slower, even though the effort feels similar or even greater.
This happens because the relationship between external workload and physiological response changes.
In other words, running at 5:00 min/km does not necessarily represent the same physiological challenge in São Paulo, near sea level, and in a city located thousands of meters above sea level.
For this reason, relying only on pace or power may be inappropriate during the first days of exposure. Perceived exertion, heart rate, and, when available, oxygen saturation can help provide a better understanding of the individual's response.
Does Higher Altitude Always Mean a Greater Challenge?
Generally, yes.
As altitude increases, the partial pressure of oxygen decreases and hypoxic stress becomes greater. As a result, the reduction in aerobic capacity tends to become more pronounced.
But there is an important distinction between acute exposure and acclimatization.
An athlete arriving at altitude for the first time does not respond in the same way as someone who has spent several weeks adapting to the environment. Acclimatization changes several respiratory, cardiovascular, and hematological responses.
This ability to adapt is precisely what makes altitude such an interesting topic in sports science.
What Does This Mean for Athletes?
If an important race or training session takes place at altitude, expecting the body to maintain exactly the same pace as at sea level can be a mistake.
Several factors need to be considered:
1. Altitude: The higher the altitude, the greater the potential impact on oxygen availability.
2. Exposure time: The first few days are different from longer periods of acclimatization.
3. Exercise intensity: The greater the aerobic demand, the greater the potential impact of hypoxia.
4. Pacing strategy: Managing effort may be more important than trying to reproduce sea-level training paces exactly.
5. Hydration and nutrition: These should be planned according to environmental conditions and the specific demands of the activity.
6. Individual response: Two athletes at the same altitude may experience very different physiological responses.
The Main Takeaway
Altitude does not simply change the scenery of a race. It changes exercise physiology.
With less pressure available to drive oxygen into the body, the organism has to work harder to maintain oxygen delivery to the muscles. Ventilation increases, heart rate changes, VO₂max decreases, and the ability to sustain certain intensities may be reduced.
At the same time, prolonged exposure triggers acclimatization mechanisms that help the body cope with the environment.
It is precisely this combination of acute performance decline and physiological adaptation that makes altitude one of the most interesting environments for studying human performance.
For athletes, the main lesson is simple: at altitude, the same pace does not necessarily mean the same effort.
