why do athletes always practise in areas of high altitude?

Question: why do athletes always practise in areas of high altitude?
- Keywords:
  athletes,
  high altitude
Asked by 398ylwj1723 to Byron, David, Esther, Henry, Jacquie on 12 Jul 2017.
- Byron Bitanihirwe answered on 12 Jul 2017:
  
  By training at high altitudes, athletes aim to allow their bodies to produce extra red blood cells which can aid in oxygen delivery to the muscles. Then, they head to a competition at lower elevations to take advantage of their changed physiology.

Question: why do athletes always practise in areas of high altitude?