High altitude does indeed make training more strenuous. Actually anyone who moves from a lower altitude to a higher altitude deals with physical changes that are strenuous on the body. Higher altitude raises your respiratory rate so the speed at which your heart beats becomes elevated. There is less oxygen in the atmosphere in higher elevations, so your body has to adjust. This adjustment takes energy and, in a training athlete, makes a person’s body work harder.
The previous stated physical changes are considered short term. These changes occur almost immediately. There are also long term changes.
- Your maximum heart rate and maximum cardiac output will decrease.
- Your red blood cells will increase in number.
- Your kidneys will work in overtime excreting a base to balance the acid in the body.
- Your red blood cells become better at giving tissues more oxygen.
- Good enzymes, such as mitochondria, will increase.
Some of these changes are of immediate benefit to athletes and some allow the athletes to work on endurance while the changes are taking place. This is why athletes like training at high altitudes.
For details: http://www.untamedscience.com/biology/human/circulatory-system/altitude/