This is my latest article as published on http://www.triathletemag.com… read on for the content.
I have had a lot of reasons to learn about altitude, most of which revolve around my struggle to find good performances at elevation. Over the years I have learned largely from trial and error out of necessity given the XTERRA tour culminates with an event in Lake Tahoe that starts at 6,300 feet and going up to about 8,800 feet. 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.
Not all athletes get the results they are looking for in implementing altitude training to their programs. Reasons may be either improper training at altitude, not managing their recovery well or because they just would not respond to altitude. I have always been interested in why there would be individual differences in altitude response. I have referenced two texts to learn about specific physiology related to altitude exposure and two papers, one that dealt with genetic differences in altitude response and the other on specific altitude responses in athletes in my research on the subject.
Altitude has become an increasingly controversial topic related to performance enhancement. Training at altitude is hypothesized to mimic the effects of doping with recombinant human erythropoietin since the body is stimulated to produce more natural erythropoietin at high altitudes. I will discuss the body’s response to altitude and how this affects VO2 max, maximal oxygen uptake, a measure of fitness that is greatly affected at high altitudes and whether performance enhancement is possible through altitude training. Hold tight – this article has lots of science words in it but if you can get through it you will have a better understanding of why you are so tired after walking up a flight of stairs in your condominium the next time you visit Vail!
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
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, PO2 (the number of oxygen molecules in a given volume of air), is less at altitude, more breaths of air must be inspired to get the same amount of oxygen. The body responds to decreased arterial PO2 by taking in more volume of air through hyperventilation. Hyperventilation decreases the partial pressure of CO2, PCO2, 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. Normally, the kidneys release bicarbonate ions to the blood to buffer carbonic acid caused by the PCO2 in the blood. When the PCO2 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 produced1.
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.
For a person at rest at sea level the amount of O2 entering the blood is determined by alveolar PO2 and the rate of blood flow in the capillaries. The PO2 at sea level is 159 mmHg and the PO2 at 8,000 feet is 125 mmHg. Hemoglobin saturation at sea level is approximately 98% and will drop to 92% at 8,000 feet. So there is a drop in oxygen carrying capacity in red blood cells at altitude. However, a 15% drop in VO2 max is normally witnessed at altitude which is not fully explained by lower hemoglobin saturation1.
The greater difference is during oxygen exchange from the blood to the muscles. At sea level, at rest, arterial pressure is about 104 mmHg and the PO2 in tissues is 40 mmHg creating a normal gradient of 64 mmHg for gas exchange to take place between the blood and the tissue. At 8,000 feet, the arterial PO2 drops to 60 mmHg and the PO2 in the tissue remains the same at 40 mmHg which creates a gradient of only 20 mmHg, a 70% decrease in pressure gradient. This greatly reduces blood to tissue gas exchange. With the reduced saturation of the cells in the blood, this compounded reduction of diffusion and oxygen transport results in a 15% decrease in VO2 max at altitude1.
It has been shown that VO2 max decreases very little at altitude until the PO2 reaches 125 mmHg (at approx 1,600m). However, VO2 max decreases exponentially with a decrease in barometric pressure at increasing altitudes1. 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. Luckily, the drop in VO2 affects all athletes the same but this is a very significant factor for, at the highest level, medal positions may reflect a difference of 1-2% in fitness. 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 total blood volume due to more red blood cells and more plasma. This allows the body to compensate for reduced PO2 since the increased blood volume increases the 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 is because oxidation is limited at lower PO2. 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.
These metabolic changes would influence muscle adaptations at altitude. Myoglobin, an iron containing protein found in the muscles that bonds reversibly with oxygen, is increased2. This protein is often found 90% saturated in the muscles and will not release this oxygen unless the Po2 in the muscles is very low. The increased desaturation in tissue seen at altitude is believed to influence myoglobin production.
An increase in capillary density in the muscles also results, reducing the diffusion distance between the capillaries and muscles. This is mostly facilitated by muscle wasting, as the number of capillaries is not reduced. A decrease in appetite and increase in caloric requirement at elevation seems to facilitate this muscle wasting if specific nutritional requirements are not met. It is important to pay special attention to nutrition at altitude because not all weight loss is what you are looking for. You are likely losing muscle as well.
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.
Conclusion
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.
Although the normal expected response to altitude has been outlined here, individuals may experience no response to altitude or a more severe response. This has been illustrated time and again by the unpredictable success rate of athletes and aspiring high altitude climbers. VO2 max seems to be a reasonable prediction of success at elevation but it is not the ultimate factor determining success.
A study has shown that native Tibetan infants, who ancestors arrived in Tibet 25,000 years ago, are better able to manage adequate oxygenation at high altitude than Han infants, whose ancestors arrived in 19513. This shows that genetics can confer advantages at altitude that prolonged exposure may not be able to achieve. This explains the success of Tibetan sherpas who have assisted western climbers on
Exposure to altitude may result in small improvements in aerobic fitness through increased oxygen transport and decreased lactate production for short time, which may make the 1% difference required for top performance at sea level. If you are a person who responds to altitude, this could be a valuable tool. At the very least, understanding altitude and its effects can help you to prepare for your next event at altitude.
References
- 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:
- Allan G. Hahn and Christopher J. Gore. 2001 “The Effect of Altitude on Cycling Performance” Sports Med Volume 31 (7): 533-557
- Susan Niermeyer, M.D., Ping Yang, M.D., Shanmina, M.D., Drolkar, M.D., Jianguo Zhuang, M.D., and Lorna G. Moore, Ph.D. 1995 “Arterial Oxygen Saturation in Tibetan and Han Infants Born in Lhasa, Tibet” New England Journal of Med Volume 333(19):1248-1252
- David F. Moffett, Stacia B. Moffett, Charles L. Schauf. Human Physiology Foundations & Frontiers, p. 585, Mosby-Year Book, Inc: