Take the same athlete to a higher stadium and two changes arrive together. The body has less oxygen pressure to work with, while the surrounding air offers less resistance. Whether performance improves or deteriorates depends on the event.
That combination explains why altitude is not simply an advantage or a disadvantage. A marathon runner and a sprinter can experience the same atmosphere very differently.
The oxygen percentage is not the main change
Air at ordinary sporting altitudes still contains roughly the same proportion of oxygen. Atmospheric pressure falls with elevation, however, so the partial pressure of oxygen falls too. Each breath therefore provides less favourable conditions for moving oxygen into the blood.
During sustained exercise, working muscles depend heavily on a continuing oxygen supply. A newly arrived athlete can find a familiar pace harder to maintain. Breathing and heart rate responses help compensate, but they do not make the environment identical to sea level.
Think of the difference between the percentage of seats reserved in a theatre and the total number of seats available. The proportion can remain similar while the underlying quantity changes. It is only an analogy, but it avoids the misleading idea that oxygen simply disappears from mountain air.
Lower air resistance changes the other side of the equation
Aerodynamic drag depends partly on air density. When the air is thinner, an object moving through it generally experiences less drag under otherwise comparable conditions.
That can help a fast sprint or change how far a struck ball travels. It can also alter the aerodynamic forces that make balls curve. The size of the effect depends on speed, shape and the event; it is not a universal distance bonus.
For a long endurance effort, the oxygen cost can dominate. For a very short event relying more heavily on energy already available in the muscles, reduced air resistance can be relatively more important.
Acclimatisation takes time
The body adjusts to altitude through several responses, including changes in breathing and, with sufficient exposure, oxygen-carrying capacity. These adaptations are why training camps and arrival schedules receive close attention.
But adaptation is not an instant switch, and individual responses vary. Being aerobically fit does not guarantee protection against altitude illness. The CDC notes that acclimatisation improves submaximal endurance, while maximal exercise performance at high altitude remains reduced compared with lower altitude.
A race result still has many causes
Temperature, wind, pacing, equipment, training and competition all matter. A fast result at elevation is not proof that altitude alone caused it; a disappointing one is not proof that an athlete failed to prepare.
Altitude changes both the engine and the resistance it must overcome. Understanding which constraint matters most in a particular sport turns an apparently contradictory story into a straightforward trade-off.