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When sporting events take place at high altitudes, the media often focus on how performance is affected by the environmental conditions. Because of the lower air resistance, explosive performances (jumping, short sprints, shot put, etc.) may actually improve. However, endurance events that are dependent on the ability to use oxygen in energy production can be hampered by high altitudes. This impact on performance is caused by the reduced barometric pressure leading to a lower pressure driving oxygen into arterial blood. There is a great deal of variation in how people respond to altitude exposure, so these comments are general observations.
The percentage of oxygen in the air is 20.93% regardless of whether you are at sea level or at high elevation. However, barometric pressure falls as altitude increases. This decrease in pressure reduces the amount of oxygen that enters the arterial blood. This decline in the arterial blood's oxygen levels means that there is less oxygen available for aerobic energy production. It has been estimated that maximal oxygen consumption (V˙O2max) is reduced by about 20% at 10,000 feet. Thus, aerobic performance is severely impaired at high altitudes.
When people first arrive at altitude, they often notice that they breathe heavier and that tasks seem to take more effort. To help offset the lower arterial oxygen levels, heart rate and breathing will be higher when performing the same task at high altitude compared with those at sea level. These physiological adjustments are the body's way of providing the muscles with more oxygen. An unacclimatized runner will have to slow his/her pace when running at altitude to stay in the target heart rate zone.
When one is chronically exposed to low oxygen pressure (hypoxia), the body responds by releasing the hormone erythropoietin. Erythropoietin causes the body to make more red blood cells. This is an important adaptation because red blood cells are responsible for transporting oxygen in the blood. The additional red blood cells help overcome the hypoxia by increasing the body's ability to deliver oxygen to the working muscles. Changes within skeletal muscle, such as increased mitochondrial density, also may be important adaptations.
An athlete who is going to compete in a high-altitude environment should give himself/herself time to adapt before the competition. Although complete physiological adaptation may take longer, a general rule is that 2 weeks are needed to acclimatize to an altitude of 7,500 feet. Additional time is needed to acclimatize to higher altitudes.
LIVE HIGH, TRAIN LOW
Endurance athletes have attempted to use the adaptations that come with chronic hypoxia to improve performance. The theory behind "live high, train low" is that an individual who remains at sea level can be exposed to hypoxia during rest periods (e.g., sleeping in hypoxic environment), but can still engage in high-intensity training; the body will acclimatize to the hypoxia and also adapt to the vigorous training. The results of such training routines are controversial. Although some have shown improved performance by such manipulations, others have not. Despite this, companies have developed various pieces of equipment to simulate high altitude (e.g., altitude tent for sleeping). At present, there is no ban on such practices, but debate continues on the benefit.
Brought to you by the American College of Sports Medicine www.acsm.org