The challenge of the heat wave: carefully manage sport, fatigue and heat

That’s it, we experienced the first strong heat of the year. Unsurprisingly, according to Météo-France forecasts, others, longer and stronger, are still awaiting us this summer. Some try to prepare for it psychologically… But what about physically?

Attention, here there is no question of the famous “summer body”, nor of method to have projecting transverses on the beach. It’s too late for that… but there’s still time to prepare physiologically for the heat waves to come. If you want to continue your favorite physical activity to keep in shape even in hot weather, know that it is possible with good preparation and a few precautions!

Even if their effectiveness fluctuates according to the sexes and the individuals, we all have mechanisms of “thermoregulation” (control of our body temperature) in order to maintain the sacrosanct 37°C:

• To fight against the cold, we can of course cover ourselves. But our body’s main strategy is the thrill. Muscle contraction only having an efficiency of about 25%, a lot of energy is dissipated in the form of heat…
• To fight against the heat, it is among other things the evaporation of our sweat that cools the skin. By conduction, the blood circulating in this skin region is cooled and returns cooler to the heart.

When these thermoregulation mechanisms are not sufficient, the body temperature changes: if it drops too much, we speak of hypothermia, and in the opposite case, of hyperthermia. The consequences are not at all the same… While the body can withstand a 9°C drop in temperature (stage 2 hypothermia) without major risk, a rise of only 3°C (temperature >40°C) can cause irreversible organ damage.

Hence the prevention messages and recommendations from the Ministry of Health during heat waves. But how to prepare for it? How can we help our body to better acclimatize to these high temperatures?

For further

The consequences of fatigue and heat stress

It should be noted that thermal stress is linked to several phenomena, internal and external. In fact, the production of heat released during muscle contractions is combined with:

• The temperature of our environment,
• Humidity which limits the evaporation of sweat, and therefore our ability to thermoregulate, beyond a certain threshold;
• The speed of the wind which impacts the ability of our sweat to evaporate.
• The radiation from the sun that warms our skin.

These parameters are used to quantify the thermal stress of an environment by calculating the “wet bulb globe temperature” index.

This heat, as we have seen, is difficult for our bodies to bear. It wears us out, literally, in many ways and it’s important to understand how.

All activity is two-way: our brain controls our muscles, and our muscles inform our brain of their physiological state. Also, fatigue following an effort is a phenomenon of both central and peripheral origin: either, respectively, at the level of the spinal cord or more (spinal or supra-spinal) – it will alter the ability of the nervous system to properly activate our muscles; and at the level of the muscle fibers – it will limit their ability to contract intensely.

The increase in our body temperature thus leads to a premature appearance of fatigue of central origin and to an alteration of our physical and cognitive performance (thinking, solving various tasks, etc.).

Physical activity and heat stress: received ideas

We often hear that physical exercise should be avoided when the outside temperature exceeds 32°C. However, we have just seen that thermal stress does not depend solely on the value indicated on the thermometer: the associated risks depend mainly on the intensity of the activity carried out.

Indeed, during an exercise, the production of internal temperature is determined by the work of our muscles: it is therefore proportional to the intensity of the effort provided. In other words, continuing to run under high heat stress is possible…if we decrease our running speed.

Another mistake is to think that only fragile people (children and the elderly in particular) are vulnerable to heatstroke. In fact, multiple factors determine this risk. Even individually, a level of heat stress can be supported once and overwhelm us on another occasion (depending on our fatigue, etc.): its occurrence is variable!

Remaining vigilant is therefore essential. How to prepare well.

How to adapt to play sports in summer?

It is possible to continue training while avoiding heatstroke. A key approach is to give yourself time to acclimatize to the heat, to go gradually.

Exposed to continuous thermal stress, our body acclimatizes in about ten days thanks to various physiological adaptations. Heart rate and body temperature, both at rest and during exercise, decrease, plasma volume (part of water in our blood) increases. We therefore sweat more and evacuate heat better.

Many athletes use preacclimatization strategies: they will solicit their physiological mechanisms of natural acclimatization to heat stress upstream. This is the case, for example, of athletes wishing to prepare for the Marathon des Sables, 240 km to be completed in five days in the Sahara desert in Morocco, where temperatures are around 50°C. This race is considered the most difficult there is.

The strategies are diverse and varied: in a hot room, in a sauna, in a hot bath, etc. The common point of all these methods is to regularly confront thermal stress so that the body adapts.

To be effective, they must follow several recommendations: choose the climatic conditions (temperature and humidity) with which the person will be confronted, have an increase in body temperature during the sessions of at least 1°C (which is greater than 38.5 °C, but below 39.7°C), maintain this increase for at least one hour and perform this for a minimum of six consecutive days.

But access to the infrastructures making it possible to simulate this thermal stress is not necessarily obvious. However, it is possible to prepare at home.

Immersion in a hot bath after exercise: a solution to acclimatize

We are currently working on a simple acclimatization method, which does not require infrastructure reproducing heat stress since it only involves taking a post-exercise hot bath:

• Jog for 40 minutes at a moderate pace, in a neutral thermal environment (~20-25°C), which raises the body’s temperature on average to around 38.5°C.
• After this exercise, immersing yourself in a bath at 40°C for 40 minutes keeps your body temperature above the threshold of 38.5°C. Be careful not to exceed 40°C, which can be dangerous. A follow-up by professionals during its first acclimatization tests is strongly recommended.
• Reproduce this sequence of jogging – hot bath six days in a row to benefit from all the physiological adaptations of natural acclimatization.

Our research has shown that this method also makes it possible to reduce the fatigue of central origin initially exacerbated during exercise under heat stress, and to reduce the impact on cognitive performance.

This method, simple to set up, could make it possible to anticipate the physiological adaptations of natural acclimatization before the arrival of high temperatures, to prepare for physical effort in a hot country before moving (sports competition in a country or travel in the middle of summer with hiking on the program) or, even, to prepare military populations for heat stress before deployment.

Ultimately, by adopting preacclimatization strategies, it is possible to prepare effectively for heat stress. We thus delay the appearance of fatigue of central origin and preserve our physical and cognitive abilities. However, it is sometimes difficult to anticipate thermal stress (lack of information, precise forecasts or unforeseen departure)…

Above all, our capacities for acclimatization (and therefore preparation) are limited: sauna like hot baths will not be enough if we have to face episodes of heat of 45°C. For our own health, we should therefore also work to limit the production of environmental heat and temperature rises.

Hervé Di Domenico, PhD student in sports sciences, Inter-university Laboratory of Motor Biology, Savoy Mont Blanc University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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