Competing at altitude helps some athletics events and harms others. Both effects come from the same fact, which is that air at elevation is less dense.
Lower density means less resistance
Air resistance depends on the density of the air being pushed aside. At elevation there is less of it per unit volume, so a moving athlete meets less drag.
For sprints and horizontal jumps, where drag is a meaningful cost and the effort lasts seconds, the reduction produces measurably better performances.
Throwing events are affected too, since an implement travelling through thinner air holds its velocity longer. The aerodynamic implements behave less predictably, because thinner air also generates less of the lift that extends their flight.
The same thinness reduces oxygen delivery
Lower density also means fewer oxygen molecules per breath, so the amount of oxygen reaching working muscle at a given breathing rate falls.
Events lasting long enough to depend on aerobic energy supply are therefore compromised, and the penalty grows with the duration of the event.
Short sprints are almost unaffected because they run on stored energy within the muscle rather than on oxygen delivered during the race. The oxygen debt is repaid afterwards, when the result has already been decided.
The crossover falls in the middle distances
There is a point at which the aerodynamic gain and the oxygen penalty offset each other, and it sits somewhere among the middle-distance events.
Below that point athletes perform better at altitude; above it they perform worse, and by the marathon the deficit is severe.
Exactly where the crossover lies depends on the individual and the elevation, which is why predictions for those events are unreliable.
Acclimatisation changes the picture over weeks
Extended exposure to altitude prompts adaptations that improve oxygen carrying capacity, which is why distance athletes train at elevation deliberately.
Those adaptations take weeks and are partly lost on return, so the timing of a descent before competition is a planning decision in itself.
Training high and competing low is the common approach, since it seeks the adaptation without the performance penalty of racing in thin air.
Records are treated inconsistently
Wind assistance invalidates a sprint record while altitude assistance does not, even though both work by reducing the resistance an athlete encounters.
Altitude-aided marks are sometimes noted separately in statistical listings, but they remain valid for record purposes and appear in the record books without qualification.
The inconsistency persists largely because excluding them would disqualify performances at venues that have hosted major championships, which the sport has been unwilling to do.
