This is an excellent excerpt reprinted from Burke's book with permission with permission from Human Kinetics, High-Tech Cycling-2nd Edition.

"Human physiology is affected in different ways at high altitude. In general, the various systems of the human body—pulmonary, cardiovascular, endocrine, skeletal muscles—respond and adjust in an effort to provide enough oxygen to survive in the hypoxic environment of high altitude. Some of these life-supporting physiological responses may also enhance athletic performance, particularly in endurance sports.

Hematological
The scientific rationale for using altitude training for the enhancement of aerobic performance is based on the body’s response to changes in the partial pressure of inspired oxygen (PIO2) and the partial pressure of oxygen in the arterial blood (PaO2). PIO2 at sea level is equal to 149 mmHg. At Mexico City (2300 m, 7544 ft), PIO2 drops to approximately 123 mmHg. At the summit of Mt. Everest (8852 m, 29,035 ft), PIO2 is approximately 50 mmHg or only about 30% of sea level PIO2.

For some reason, triathlon attracts many who want to dig into the science of how to train, researching questions like, "Why do I need long runs AND short fast runs?" "Why should I train my core so much if I am not in a sit-up competition?" "Swimming is really the only technique-oriented sport, right?"

It all starts with base training. Marc Evans writes about base training in Triathlete's Edge. The following is an excerpt from his book reprinted here with permission from Human Kinetics.

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