FWIW:
As altitude increases, the sustainable rate of work will decrease (more anaerobic work). This will vary from person to person depending on genetics, altitude exposure, etc...
Ze Mane:
Exactly. Here is the percentage of oxygen in the air (as compared to sea level) at these altitudes:
8000 ft 76%
10000 ft 70%
12000 ft 65%
14000 ft 61%
As a consequence of the lower amount of oxygen in each breath:
1. the body with have to use more anaerobic power to maintain power levels
2. the heart rate will increase in an effort to supply more blood flow to the working muscles
