Heat, altitude: challenges and opportunities of environmental constraints for athletes
How hot will it be at the Paris Olympics in 2024? With global warming, the number of sporting competitions taking place during summer heat waves is increasing. Athletes' exposure to heat is also increasing with the emergence of new nations hosting major competitions in hot regions, such as Qatar for the World Athletics Championships in 2019, the United Arab Emirates with the final of the World Triathlon Championship Series 2022, or Senegal with the Youth Olympic Games in Dakar in 2026.
Jonathan Rubio, University of Montpellier
While specific protocols and regulations are beginning to emerge within the various international federations, enabling events to be cancelled or modified in the event of extreme conditions (the latest example being the Rugby World Cup with the France-New Zealand match, for which cool-down breaks were held in the middle of both periods), athletes must also prepare specifically for the heat, on pain of performance impairment in endurance disciplines, or even medical problems leading to death from heatstroke in some cases.
When preparing for a major sporting event, endurance athletes such as triathletes are accustomed to spending several weeks at altitude: since the 1968 Olympic Games in Mexico City, the benefits of altitude have been scientifically documented. The main effect is an improvement in performance via an increase in haemoglobin mass, enabling oxygen to be transported more efficiently throughout the body and to the muscles, thereby contributing to the latter's energy production.
However, before a competition in a hot environment, it is necessary to make another specific preparation, involving a stay in this same type of condition, or at least training, which not only enables the athlete to cope with the heat in the best possible way, but also protects his or her health.
Environmental stresses in training can be beneficial... if used properly
Incorporating environmental stresses such as heat or altitude into training plans is not without consequences. In fact, when coaches plan training courses for athletes by adding environmental stress, it adds to all other types of stress, especially those related to training load. For example, the presence of too much heat or hypoxia stress without adjusting training intensities and volume can undermine the expected benefits. Too much physiological stress leads to fatigue, preventing the body from adapting.
Today, to ensure that preparation for competitions runs smoothly, athletes are closely monitored, recording their condition by means of various physiological parameters.
Training for competitions in warm conditions
When it comes to heat, athletes train in natural or controlled conditions. In other words, in a room specially designed for warm environments, in which the athletes carry out their exercises, ranging from weight training to home trainer cycling or treadmill sessions.
It quantifies the desired heat stress, i.e. both target temperature and humidity (usually around 38°C and 50% humidity). The recommended daily dose of heat is relatively low, on the order of 60 to 90 minutes per day for daily training.
Athletes are fitted with capsules which they swallow several hours beforehand to monitor their body temperature in real time. This enables us to better control the risk of heatstroke, and ensure that the hyperthermia required for cellular adaptation is present (the goal is to reach 38.5°C internal temperature for more than 30 minutes).
Other physiological parameters specific to heat acclimatization are monitored, such as sweat output, urinary density to check for excessive dehydration, electrolyte losses ([Na+], useful information particularly in the case of long-distance events, to compensate for losses by enriching sodium intake), hematocrit (volume of red blood cells in the blood), and exercise heart rate, the latter reflecting the overall state of acclimatization.
Perceptual markers based on sensory scales of comfort and thermal sensation are also provided for each session.
Training with less oxygen: a textbook case of the need to individualize protocols
The dosage of hypoxia training is far more complex and requires far more care than for heat to achieve results, especially in endurance athletes who already have a very high haemoglobin mass. While in the case of heat, exposure time is limited, the aim is to spend much longer in hypoxia.
In the past, we tended to speak of "responder" or "non-responder" athletes to hypoxic stress (i.e., some don't produce extra red blood cells after being exposed to a hypoxic environment), whereas today we know that a non-responder athlete at a certain altitude and training level can become one by changing the dose of hypoxia, i.e., either by increasing the exposure time at the same altitude, or with a similar exposure time but at a higher altitude. But it's not all plain sailing... some people need a higher dose, others a lower one.
This individual adaptation of hypoxic stress conditions optimizes erythropoietic responses (i.e. red blood cell production). With this in mind, endurance athletes can now sleep at a higher altitude than their home base, using hypoxic chambers. Constant adjustments are made to the altitude regulation of the chambers to control the amount of daily hypoxic stress induced, so as to best control the athletes' fatigue levels.
The latter benefit from equally specific monitoring, notably with pulse oximeters that collect their O2 saturation data during the night. In the case of preparation for a competition just after the course, for example, it is estimated that an average oxygen saturation of 90-91% (compared with 100% at sea level) is sufficient to induce altitude-related adaptations without generating excessive fatigue.
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During the acclimatization phase (between 3 and 8 days), training intensity is minimized. Overall fatigue is assessed each morning, with heart rate variability analyzed, enabling the day's training loads to be adjusted if necessary. To check athletes' hydration status, the same procedure as for heat is carried out at urine level, combined with a morning weighing. Athletes are monitored by means of fitness and altitude-sickness questionnaires. Their blood sugar levels are also monitored using patches glued to the skin, to determine whether athletes are experiencing nocturnal hypoglycemia over a long period, which can lead to fatigue during the course. This makes it possible to adjust their diet by changing the composition of the sugars taken at the last meal.
Key adaptation points and outlook
Ultimately, the question of training dosage in relation to the dose of environmental stress is a key point in the success of training courses. The physiological response of each athlete is taken into account to ensure that the body adapts well without tiring too much.
Individualized dosing is essential if elite athletes are to derive maximum benefit from their performance.
Finally, the interest of environmental stresses of hypoxia and heat is that they induce physiological responses of interest for endurance performance (an increase in the number of red blood cells for hypoxia and an increase in cardiac contraction thanks to a rise in plasma volume for heat). The idea of combining these two stresses in order to couple the benefits of each is tempting. New research conducted by CREPS Montpellier is currently underway on this subject.
This article is published as part of the Fête de la science (which takes place from October 6 to 16, 2023 in mainland France, and from November 10 to 27, 2023 overseas and internationally), and of which The Conversation France is a partner. The theme of this year's event is "sport and science". Find all the events in your region on the Fetedelascience.fr website.
Jonathan Rubio, PhD student in exercise physiology , University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.