When altitude increases during exercise the pO2 (partial oxygen pressure) decreases. With every breath you will get less oxygen in your lungs, which also means that there is less oxygen in your blood that can be transported to your muscles.
Your hearth rate will increase so more oxygen can be transported to your muscles. This means that the heart quickly reaches its maximum output and your threshold power decreases. At altitude, recovery after exercise also slows down.
Power output decreases at altitude
When altitude increases, your power output decreases at the same relative intensity. Various models try to estimate the theoretical decrease in aerobic power output1 2. These models are based on well-trained athletes. When athletes stay at altitude for at least two weeks, their body adapts to the thinner air which leads to a lower decrease in power output.
The summit of the Alpe d'Huez is situated at 1850m, which means that the last 4 km of the climb are at an altitude of 1500m and higher. When your threshold power at sea level is 250 Watts, it will be about 8,7% lower (228 Watts) at 1500m. This also means that your power zones change.
VO2max also decreases at altitude
Several scientific studies show that VO2max also decreases when altitude increases. The exact decrease is not clear because there are many differences between the study design and the subjects involved in the different studies.
Compared to values at sea level, a small decrease in VO2max has already been observed at an altitude of 589m. If altitude increases, the amount of oxygen that can be absorbed into the blood decreases, causing the VO2max of men and women to decrease about 7-9% per 1000m3. Between individuals, the decrease in VO2max can significantly differ4.
Will your performance also decrease?
To determine performance, gross efficiency, pacing strategy, and aerodynamics are, in addition to power and VO2max, also important. Research shows that altitude doesn’t affects gross efficiëncy2. At altitude, air resistance is lower, which means that you have to produce less power to attain a certain velocity.
The question is whether power loss is sufficiently compensated by the reduced air resistance. Research shows that performance over 1000m decreases by 2-4% in medium and long distance runners and above 2000m with> 4%6.
When this is translated into cycling, you could theoretically be 2-4% faster on a climb that starts at sea level compared to the same climb if it would start at an altitude of 1000m.
1. Basset, D., Kyle, C., Passfield, L., Broker, J., & Burke, E. (1999). Comparing cycling world hour records, 1967-1996: modeling with empirical data. Medicine and science in sports and exercise, 31(11):1665-76.
2. Péronnet, F., Thibault, G., & Cousineau, D. (1991). A theoretical analysis of the effect of altitude on running performance. Journal of applied physiology, 70(1):399-404.
3. Fulco, C., Rock, P., & Cymerman, A. (1998). Maximal and submaximal exercise performance at altitude. Aviation, space, and environmental medicine, 69:793.
4. Robergs, R., Quintana, R., Parker, D., & Frankel, C. (1998). Multiple variables explain the variability in the decrement in VO2max during acute hypobaric hypoxia. Medicine and science in sports and exercise, 30:869.
5. Clark, S., Bourdon, P., Schmidt, W., Singh, B., Cable, G., Onus, K., Woolford, S., Stanef, T., Gore, D., Aughey, R. (2007). The effect of acute simulated moderate altitude on power, performance and pacing strategies in well-trained cyclists. European journal of applied physiology, 102(1):45-55.
6. Hamlin, M., Hopkins, W., & Hollings, S. (2015). Effects of altitude on performance of elite track-and-field athletes. International journal of sports physiology and performance., 10(7):881-7.