Welcome to the first article of an exciting two-part series exploring how human physiology is altered while at altitude, and how altitude training can be utilised to improve aerobic performance. This article will explain at how the body responds when exercising at altitude and what consequences it could have on performance.
What is the challenge of altitude?
It is a well-established fact that as elevation increases, the density of the air decreases. This doesn’t have any effect on the proportion of oxygen that is in the air (that always remains at about 20.9%), but it does mean that there is less oxygen available to breathe in. As a result, less oxygen enters the body through the lungs so there is a reduction in the amount that is delivered to our working muscles, causing a large disruption to homeostasis. This effect worsens the higher up you venture, making it increasingly more difficult to achieve an optimal performance. Surprisingly, reductions in VO2max can be seen at altitudes as low as 600m above sea level.1
What adjustments does the body make to cope with the reduced oxygen availability?
Upon immediate exposure to altitude, peripheral and central chemoreceptors (receptors around the body that detect changes in the composition of the blood) notice that there is a lower concentration of oxygen in the arterial blood than usual. The activation of these receptors drives an increase in ventilation in an attempt to increase the amount of oxygen that can enter the body and be delivered to working muscles.2
The increase in ventilation does not fully compensate for the drop in oxygen content of the blood, therefore other changes have to be made in order to maintain sufficient oxygen delivery to working muscles. Another way to do this is to increase cardiac output, which is the amount of blood that the heart can pump out per unit of time. In this case, the change in cardiac output is achieved by increasing heart rate.3
What are the impacts of these adjustments?
There are important consequences that follow from the increase in ventilation that is immediately experienced when exposed to altitude. By ventilating more, there is a large volume of carbon dioxide expelled from the body. This causes the pH of the blood to become significantly more alkaline. In order to restore the normal blood pH, actions from the kidneys increase excretion of bicarbonate, a metabolic by-product used to regulate acid balance of the blood. This response means that the blood is less-able to buffer the metabolic products from exercise, such as lactate, so high intensity exercise becomes much more difficult.1
Another factor to consider when exercising at altitude is the risk of dehydration. There are several factors that may lead to this, the first being that the increase in ventilation results in greater water loss.2 The second is that along with the excretion of bicarbonate, diuresis occurs (increased urine production), again resulting in large fluid loss from the body.1 Finally, when at altitude, a loss of appetite can occur that may be accompanied by nausea, therefore causing a lack of thirst.2 The combination of these factors poses a significant risk, and can often result in the reduction of plasma volume in the blood. Particularly when exercising in environments that are both hot and mountainous (a common occurrence in road cycling), special care must be taken to ensure that an athlete does not become critically dehydrated.
What effect does this have on performance?
The physiological responses previously discussed are not able to fully compensate for the reduced oxygen content of the blood. As well as this, the decreased blood buffering capability means that there is a greater lactate response. VO2max declines as a result, as it becomes significantly more difficult to reach the optimal levels of performance.3 This effect worsens with increasing altitude; VO2max is reduced by 1-2% with every 100m above an altitude of 1500m.1
At submaximal workloads, the same oxygen uptake (VO2) can be achieved at altitude than at sea level because of the increased ventilation and heart rate responses to increase oxygen delivery. However, the relative intensity of a given workload is greater than it would be at sea level, because is now a greater proportion of (the now reduced) VO2max.3
To summarise what this means, exercise at altitude results in a greater physiological stress on the system and higher ratings of perceived exertion compared to exercise at sea level, with a greater effect as elevation increases.1
Stay tuned for part two of Exercise at Altitude, exploring the physiological mechanisms behind altitude training, and how it may be effective to improve aerobic performance!
References:
- Drust B, Waterhouse J. Exercise at altitude. Scott Med J. 2010;55(2):31‐34. doi:10.1258/rsmsmj.55.2.31
- Institute of Medicine (US) Committee on Military Nutrition Research, Marriott BM, Carlson SJ, eds. Nutritional Needs In Cold And In High-Altitude Environments: Applications for Military Personnel in Field Operations. Washington (DC): National Academies Press (US); 1996.
- Mazzeo RS. Physiological responses to exercise at altitude : an update. Sports Med. 2008;38(1):1‐8. doi:10.2165/00007256-200838010-00001
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