Competing at altitude affects events in opposite directions. The same reduction in air density that assists a sprinter works against a distance runner.
Air density falls with elevation
Atmospheric pressure decreases with height, so a given volume of air contains fewer molecules than the same volume at sea level.
Fewer molecules mean less resistance to a body moving through the air, and also fewer oxygen molecules in each breath taken.
Both consequences follow from the same physical fact, which is why altitude cannot help one kind of athlete without hindering another. The change is gradual rather than sudden, so the size of the effect depends on how high the venue sits.
Why sprinters and jumpers benefit
Short events are limited by power output against resistance rather than by oxygen supply, since the energy used is stored in the muscle rather than delivered by breathing.
Reduced drag therefore translates almost directly into faster times, and the effect grows with speed, making it largest in the shortest events.
Horizontal jumps gain twice over, from a faster run-up and from reduced resistance during flight, which is why altitude marks in those events stand out. Throws behave similarly, since an implement travelling through thinner air loses less speed on the way down.
Why distance running suffers
Endurance performance depends on delivering oxygen to working muscle, and thinner air reduces the amount available in each breath.
The body compensates by breathing harder and raising heart rate, but neither fully restores the shortfall, so sustainable pace falls.
The penalty grows with race duration, which is why distance times at altitude are consistently slower while sprint times are consistently quicker.
Acclimatisation and its limits
Spending time at elevation prompts adaptations that partly offset the oxygen deficit, and athletes preparing for altitude competition arrive well in advance.
Those adaptations take weeks rather than days, and arriving shortly before competing generally produces the worst of both situations.
Adaptation never fully closes the gap, so even acclimatised distance runners race slower at height than they would at sea level. Training at altitude and competing lower down is the more common arrangement for exactly that reason.
How records account for it
Sprint and jump marks set above a defined elevation are noted as altitude-assisted, since the conditions are recognised as materially different.
Distance records carry no such note, because altitude only makes those events harder and a mark set there needs no qualification.
This asymmetry occasionally causes confusion, but it follows directly from the fact that only an advantage requires flagging, not a disadvantage.
