By Melanie McQuaid
Aug. 14, 2007 — I have had a lot of reasons to learn about altitude, most of which revolve around my struggle to perform well at elevation. Over the years I have learned largely from trial and error, but also through some research on the physiological effects of altitude. I will discuss the physiological effects of altitude on the body and in particular explore the consequence of altitude on athletic performance. The use of altitude training to improve athletic performance is widely accepted, evidenced by the massive migration of superstar athletes to Boulder, Colorado. Either by traveling to high altitude locations or by using hypoxic tents (which increase the concentration of nitrogen in the environment and thus simulate a lower partial pressure oxygen environment) athletes seek to reap the physiological benefits of altitude exposure.
Physiological Responses to Altitude
There are three main responses to altitude: respiratory, cardiovascular and metabolic responses. Although thorough research has not been completed to explain, in detail, each response within the body, a lot has been discovered since the Olympic Games in Mexico City (2,240m or 7350 feet) in 1968, where altitude was a factor in athletic performance for the first time. However, some details have yet to be discovered. Numerous studies have been completed on theories relating to how altitude can be utilized to improve sport performance and to date there is no consensus on the benefit of altitude on sport performance. However, in theory it should promote better performance, particularly in endurance sport.
Physical performance is dependent upon respiration to bring oxygen into the body, transport it via the bloodstream where it is taken up by the muscles. These three steps, pulmonary ventilation, pulmonary diffusion, oxygen transport and gas exchange at the muscles is the first and most symptomatic of responses to altitude.
The initial response to altitude, lasting 7-10 days, is an increased alveolar ventilation both at rest and while training. Because the partial pressure of oxygen (hereby referred to as PP0) is less at altitude, more breaths of air must be inspired to get the same amount of oxygen. The body responds to decreased arterial PPO by taking in more volume of air through hyperventilation. Hyperventilation decreases the partial pressure of CO2 in the alveoli of the lungs which then creates a pressure gradient in the alveoli forcing more CO2 out of the blood stream and into the lungs to be exhaled. This increased CO2 clearance allows the blood pH to increase which is known as respiratory alkalosis. When the CO2 decreases during the altitude response, there is less acid in the blood and the pH rises which limits the buffering capacity of the bloodstream. This becomes a concern during intense exercise when lactic acid is produced.
Increased ventilation also increases dehydration due to normal fluid losses during respiration. At high altitudes, the amount of water held in the air at increased barometric pressure is reduced, thus the amount of water inhaled during each breath is less which further compounds the fluid losses at high elevation.
It has been shown that VO2 max decreases very little at altitude until the PPO reaches 125 mmHg (at approx 1,600m). However, VO2 max decreases exponentially with a decrease in barometric pressure at increasing altitudes. Since VO2 max drops so drastically at very high elevations, this is why supplemental oxygen is required for mountaineers at extreme elevations. A very high VO2 max (approaching an elite athlete’s level) would be required to successfully climb Everest without oxygen for as the summit is approached, the maximal oxygen uptake drops to nearly a quarter of its original value, which leaves the climber with very little capacity to do work. If that climber started with an average VO2 max, there would be little chance they could continue when their VO2 max was decreased at high altitudes.
This decrease in VO2 max is a consideration for athletes competing at altitude. A very high VO2 max would be required for great performances at moderate altitudes as it is shown that VO2 max will decrease by 15%. Since VO2 is considered a metric for aerobic capacity, relating this to your maximum wattage on the bike may see your watts decrease from 300W at sea level to 255 W at altitude. VO2 max is already related back to your weight, so often lighter people with huge aerobic capacity will have higher VO2 numbers. However, if you consider power to weight ratio as a metric of relative fitness you will also see this value drop drastically. The only way one could counter the decrease in power to weight ratio due to decreased power would be to lose weight. However, for a 60 kg person this would be a whopping 9 kg weight loss if their max power dropped from 300W to 255W at elevation. This would likely result in further loss in power so probably would not be effective regardless. It is often seen that athletes with a lower weight, and corresponding high VO2 numbers, tend to fare better at altitude. This is probably related to a smaller decrease in overall power to weight ratio.
The second physiological response is in the cardiovascular system. In the first few weeks of exposure to altitude (a minimum elevation of 1,600 m is generally required to elicit a response in the body) the blood plasma volume decreases, which increases the number of red blood cells per unit of blood. This is the body’s attempt to increase the amount of oxygen reaching the tissues. For athletes, this decrease in plasma volume also results in a decrease in stroke volume for each cardiac output. Coupled with a decreased diffusion gradient limiting oxygen exchange at the muscles a significant decrease in performance is seen in the first 10 days at altitude.
The body’s compensation for a pressure gradient limiting O2 delivery is to increase the volume of blood available for transport. After the initial drop in plasma volume, the total number of red blood cells remains unchanged, and the resulting increase in hematocrit is reflecting the increased concentration in the bloodstream. This means that overall the volume of blood initially is reduced. The body releases erythropoietin into the bloodstream to increase reticulocytes (new red blood cells) during the first 24-72 hours at elevation2. In addition, the body then starts to produce more blood plasma to bring the volume of plasma back to normal levels. These adaptations ultimately result in an increased stroke volume of each cardiac output. This is the primary objective of athletes training at altitude. By influencing the stroke volume of each cardiac output, more oxygen is being transported to working muscles and thus an increase in VO2 max can be expected along with improved performance.
The third response involves metabolic changes at elevation. Athletes would experience more anaerobic metabolism at lower heart rates where aerobic metabolism would be expected. This would increase the amount of lactic acid produced at any given work load. This influences the volume and intensity of any training that might be done at altitude as the response is different than expected at sea level. This is particularly significant for endurance athletes who mainly compete in aerobic energy systems for longer periods of time; as they would expect to require greater recovery time because of the higher lactic acid response and also because of the decreased buffering capacity of the blood to facilitate lactic acid flushing.
An increase in anaerobic metabolism may lead to a decrease in the production of, or an increase in clearance of, lactate over the course of training 2-4 weeks. This would be a beneficial training effect for endurance athletes who train to increase lactate tolerance. This would allow athletes to maintain longer periods at lactate threshold due to increased lactate clearing and/or tolerance, which would result in better performance in high intensity endurance sport.
It has been shown that athletes require a minimum of two weeks for altitude acclimatization; however an even longer period would be required for optimal performance to be achieved. An alternate strategy would be to arrive to competition within 24 hours to minimize the adaptation response to elevation. The numerous physiological responses which occur at altitude create a great deal of stress on the body and inhibit high level performance, so a recovery period would be required to regain top athletic ability.
To use these adaptations to altitude to best advantage for endurance athletes, in theory, would require athletes to live at higher altitude but train at lower altitude. The main challenge is the detraining effect that happens if high intensity training cannot be completed because of extreme altitude. Therefore, training at a high PO2, sea level, but recovering at a low PO2, at elevation, while utilizing physiological responses to altitude is thought to be ideal. However, all of this is controversial because many studies have shown that there is considerable variation in individual response to altitude.
Based in Victoria, Canada, Melanie McQuaid is a three-time defending XTERRA world champion. For more information about McQuaid, please visit www.racergirl.com
1. Jack H. Wilmore, PhD. / David L. Costill, PhD. 1999 “Exercise in Hypobaric, Hyperbaric and Microgravity Environments” in Physiology of Sport and Exercise (second edition), pp. 343-357, edited by H. Gilly and J. Rhoda. Human Kinetics: Windsor, ON.
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