The reason is that at higher altitudes, there is lesser amount of oxygen for the body and muscles. Therefore, the excess oxygen will help avoid the early production of lactic acid and keep heart rate lower even when the athlete is working harder at sea level. (Smith, 2005)
The trick to high altitude training is a process known as acclimatization. This means that athletes must give time to their bodies to get used to the increase in altitude, and decrease in oxygen levels in the atmosphere. For instance, when an athlete reaches, say 5000 feet, he must spend some days there so that the body acclimatizes to the conditions present there before moving on to higher altitudes. After acclimatization to higher altitudes, when the athlete returns back to sea level, his endurance level and performance is better.
As the oxygen levels at higher altitudes decrease, there are a number of changes that the body undergoes in the process of acclimatization. Firstly, the depth of respiration increases. The pulmonary arteries go through an increase in pressure, forcing blood into those parts of the lung which are not utilized under normal circumstances. (Curtis, 1999)
Along with increasing the production of red blood cells to carry oxygen, the body also steps up the production of a specific enzyme that eases the discharge of oxygen from hemoglobin to the body tissue. (Curtis, 1999)
As we know, the air in the atmosphere consists of 78% nitrogen, 21% oxygen and 1% water vapor, carbon dioxide and other gases. This is the proportion of gases you inhale during normal breathing. However, 6% of oxygen is breathed out together with carbon dioxide and other waste products. (Science Fair 2003: Does Exercise Affect the Amount of Carbon Dioxide Exhaled, 2003)
During exercise, the body needs more energy which is provided to it through the chemical reaction of oxygen and glucose. This, in turn, means that during exercise, the body needs more