Acclimatization To Altitude

Acclimatization to altitude has become an important part of the preparation process for athletes competing above 1500m (4921ft).

Conditions above this level make physical activity more difficult and limits performance (2). But what is the most effective method for acclimation and can training at altitude improve performance at sea level?

This article focuses on the immediate physiological responses to a hypobaric (low atmospheric pressure) environment and the longer-term adaptations that take place in the body.

Although conditions at altitude have been known for many years, in 1968 the Olympic Games in Mexico City drew considerable attention to their specific effect on athletic performance.


High Altitude Environment

Air at altitude is commonly mistaken for being lower in oxygen but this is incorrect. Air, at any level, contains 20.93% oxygen, 0.03% carbon dioxide and 79.04% nitrogen. Instead, as elevation increases, oxygen has a progressively lower partial pressure (1)

At any point on earth, the more air that is above that point, the greater the barometric pressure will be. This is the same principle as being under water. The deeper a diver is the more water there is above her and the greater the pressure. At sea level, air exerts a pressure of approximately 760mmHg. At the summit of Mount Everest, 8848m (29,028ft) above sea level, air only exerts a pressure of about 231mmHg (2).

Recall that after we inhale, oxygen in the alveoli (tiny air sacs in the lungs) passes to the blood to be transported to the tissues. This gas exchange between the alveoli and blood takes place due to a pressure difference called a pressure gradient. The pressure oxygen exerts in the alveoli is greater than the pressure of oxygen in the blood surrounding the lungs. This drives oxygen from the lungs into the blood (1,2).

It makes sense then that any reduction in the pressure of oxygen entering the lungs will reduce the pressure difference or gradient. The result is less oxygen being driven from the lungs into the blood. At altitude that is exactly what happens.

The weight of air and the barometric pressure it exerts has an effect on the partial pressure of oxygen. At sea level, oxygen has a partial pressure of 159mmHg. In Mexico City it is approximately 125mmHg. At the top of Everest, it drops to 48mmHg, which is nearly equal to the blood surrounding the lungs (2). With very little pressure difference at this level oxygen exchange is severely hampered and its not surprising that supplemental oxygen becomes essential for most.

While there are other changes at altitude such as a drop in temperature, decreased humidity and increased solar radiation, the reduction in the partial pressure of oxygen (and so oxygen transport to the tissues) is thought to be the major cause of reduced exercise performance (2).


Acute Response to Altitude

Recall from the VO2 max article that the bodys ability to supply and utilize oxygen is a limiting factor in performance. Up to 1500m (4921ft), altitude has little effect on the body. Above this level, studies on men show the cardiovascular, respiratory and metabolic systems are affected. Unfortunately, there are few studies on women and children at altitude and their responses may differ slightly.

Respiratory System Response to Altitude


Cardiovascular System Response to Altitude


Metabolic Responses to Altitude

Lack of oxygen availability and utilization at altitude places a greater demand on anaerobic metabolism to produce energy. This results in an increase in the concentration of lactic acid at any given submaximal exercise intensity compared to sea level. In contrast, lactate concentration is lower during maximal effort (6,7).


Athletic Performance Altitude

As would be expected the acute responses mentioned above have a detrimental effect on exercise performance in particular the endurance events. VO2 max decreases significantly as altitude increases. Running at 12km/h for example will equate to a higher percentage of VO2 max when completed at altitude compared to sea level.

Conversely, anaerobic events lasting under a minute such as sprinting, throwing and jumping activities are not impaired at moderate altitude. In fact, they can actually be improved due to the thinner air and less aerodynamic resistance (2).


Acclimatization to Altitude

It takes approximately two weeks to adapt to the changes associated with the hypobaric conditions at 2268m (7500ft), roughly that of Mexico City (1). Every 610m (2000ft) increase requires an additional week of acclimatization to altitude (1). But no matter how long an individual lives at altitude, they never fully compensate for the lack of oxygen and never regain the level of aerobic power or endurance performance they could at sea level. Below are the major adaptations occur with acclimatization to altitude:

Acclimatization To Altitude



Preparing for Competition at Altitude

How can athletes who live at sea level prepare for a competition at altitude?

One approach is to compete within 24 hours of arrival at altitude. Not much acclimatization will have taken place but most of the classical symptoms of altitude sickness will not have had time to manifest. After the intial 24 hours, dehydration and sleep disturbances become more prominent.

An alternative option is to train at a higher altitude for at least 2 weeks prior to competition. Although full acclimatization to altitude takes 4 to 6 weeks, many of the physiological adaptations occur in the first 2 weeks and the more severe disturbancs should have settled. It is important to remember that during the intial days at altitude work capacity is reduced, so athletes should train at 60-70% of sea level VO2 max and build up gradually over 10-14 days.

A third approach is to devote a greater percentage of training time at sea level to endurance training several weeks prior to competition. This is a strategy often adopted within many team sports, helping to raise players' VO2 max to a peak so that they can perform at a lower relative intensity without significant loss in performance.

Sleeping in altitude tents and hypobaric chambers may be able to adequately simulate the effects of altitude but these tend to be very expensive. Unfortunately, there is no evidence to suggest that spending 1-2 hours per day breathing hypobaric gases at sea level results in the same adaptations as living at altitude.

Can altitude training improve sea level performance? See the altitude training article for more details.



Return from this acclimatization to altitude article
to the main Exercise Physiology Section



References
1) McArdle WD, Katch FI and Katch VL. (2000) Essentials of Exercise Physiology: 2nd Edition Philadelphia, PA: Lippincott Williams & Wilkins
2) Wilmore JH and Costill DL. (2005) Physiology of Sport and Exercise: 3rd Edition. Champaign, IL: Human Kinetics
3) West JB, Boyer SJ, Graber DJ, Hackett PH, Maret KH, Milledge JS, Peters RM Jr, Pizzo CJ, Samaja M, Sarnquist FH, et al. Maximal exercise at extreme altitudes on Mount Everest. J Appl Physiol. 1983 Sep;55(3):688-98
4) PUGH LG, GILL MB, LAHIRI S, MILLEDGE JS, WARD MP, WEST JB. MUSCULAR EXERCISE AT GREAT ALTITUDES. J Appl Physiol. 1964 May;19:431-40
5) Grover RF, Reeves JT, Grover EB, Leathers JE. Muscular exercise in young men native to 3,100 m altitude. J Appl Physiol. 1967 Mar;22(3):555-64
6) Green HJ, Sutton J, Young P, Cymerman A, Houston CS. Operation Everest II: muscle energetics during maximal exhaustive exercise. J Appl Physiol. 1989 Jan;66(1):142-50
7) Sutton JR, Reeves JT, Wagner PD, Groves BM, Cymerman A, Malconian MK, Rock PB, Young PM, Walter SD, Houston CS. Operation Everest II: oxygen transport during exercise at extreme simulated altitude. J Appl Physiol. 1988 Apr;64(4):1309-21
8) Wolfel EE, Groves BM, Brooks GA, Butterfield GE, Mazzeo RS, Moore LG, Sutton JR, Bender PR, Dahms TE, McCullough RE, et al Oxygen transport during steady-state submaximal exercise in chronic hypoxia. J Appl Physiol. 1991 Mar;70(3):1129-36.



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