It seems that extreme heat is a climatic phenomenon of the planet that will increase over the years. That is why it is so relevant to study the human capacity to exercise in environments at high and very high temperatures, as in Qatar 2022. Thermography allows us to measure body temperature quickly and in a non-invasively way, facilitating the assessment of players.

It is estimated that, currently, more than one billion people live in extreme temperature conditions at some point during the year (Ebi et al. 2021) and there are more than 25,000 deaths from excess heat in Europe each year (Leyk et al. 2019). In addition, acute exposure to extreme heat increases the risk of mortality from a variety of causes (Burkart et al. 2021). Global data from NASA, combining historical measurements with data from climate simulations, provide alarming forecasts of the critical situation on the planet (Figure 1).

Figure 1. Thermal image of the Earth, showing the areas of higher temperature (dark red), with worse conditions for life due to high temperatures. Extracted from NASA.gov.

Furthermore, these factors, together with the increased frequency and intensity of heat waves (Haines et al. 2006), are expected to hamper international sports competitions (Olympic Games, World Championships, etc.) with increasing restrictions on when, where, and how they can be held (Smith et al. 2016). Hence the importance of knowing the consequences and risks of training and competing in extreme heat in the lead-up to the Qatar 2022 World Cup.

Risks of training and competition in extreme heat.

Exercise in extreme heat increases body temperature and thermoregulatory stress (O'Connor et al. 2020), which can result in a variety of health and performance problems. Physical work capacity and psychomotor performance are compromised in this situation (The Lancet 2021; Ebi et al. 2021; Leyk et al. 2019), which in professional athletes impacts the final performance (Périard et al. 2021; Ball et al. 2021; McKay et al. 2021).

One of the most common risks of acute heat stress is the so-called heat stroke. The main signs and symptoms of heat stress range from muscle cramps to loss of consciousness, nausea and generalized fatigue, and can even lead to death. It is also curious that, although in a mild case of heat stroke there is a tendency for excessive sweating, as body temperature rises, sweating can stop completely, which in turn prevents heat dissipation (Leyk et al. 2019; Epstein et al. 2019), dangerously increasing body temperature. In sports, as temperatures can be even higher, there is a risk of death. This incident is almost null in professional athletes, but in recreational athletes is somewhat higher. Even if it is almost anecdotal, between 2001 and 2018, 38 deaths of athletes from overheating were recorded in Australia (Fortington et al. 2021).

If core body temperature rises above 39.5 °C, immediate cooling strategies such as immersion in cold water (<14 °C) or covered with wet towels are recommended (Leyk et al. 2019). To measure it quickly and noninvasively, we can use a thermographic camera and extract the temperature from the inner canthus of the eye (Zhou et al. 2020; Pascoe et al. 2010; Mercer et al. 2009).

Figure 2. Region of the inner canthus of the eye, where the temperature is taken for thermal analysis (A) and comparison of measurements with thermography and with axillary contact thermometer of two persons, one with fever (B) and the other without fever (C).

Figure 2 shows the region of the face where the temperature is measured in febrile states. The difference between core temperature (Tcore) and skin temperature (Tskin) is considered to be ±0.5 ºC, so if the measurement exceeds 39 ºC, it is considered dangerous due to the risk of heat stroke.

As explained in Figure 3 (Périard et al. 2021), performing physical exercise under heat stress causes thermoregulatory stress and problems due to excessive fluid loss. This strains the system and decreases performance while increasing the risk of heat stroke.

The consequences of this review are very much in line with those found in other studies, such as the one reviewed in this publication.

Figure 3. Summary of the consequences of exercise under heat stress, including exacerbating and mitigating factors. Extracted from Périard et al. 2021.

Recommendations for training and competition in extreme heat

The scientific literature has evolved a lot since the recommendations of Adolph (1947), who made very rough descriptions of the measurement of excretion and metabolic adaptations to extreme heat conditions.

Today, we know that even in moderate ambient temperatures, physical exertion can lead to heat stress, decreased performance and health risks. Moreover, the risk is higher when it involves intense metabolic heat production and/or where heat dissipation is restricted, such as performing prolonged high-intensity physical activity in extreme heat (Leyk et al. 2019).

Another aspect to take into account is that there is usually associated dehydration. During prolonged exercise, significant fluid losses are experienced through thermoregulatory sweating, which helps to control hyperthermia. However, if these losses are not replaced, performance decreases because of the overall alterations in physiological function (Trangmar et al. 2021). Therefore, monitoring of euhydration status and individualization of fluid intake in terms of quantity, frequency and type of beverages is necessary, taking into account possible variations in environmental conditions, exercise intensity and heat acclimatization status (McCubbin et al. 2020).

Although, without a doubt, it seems that the most important intervention with the aim of reducing thermoregulatory stress and optimizing performance is progressive heat acclimatization. This acclimatization period should be performed with repeated exposures to exercise and heat for 1-2 weeks, in a state of euhydration and minimizing dehydration during exercise (Racinais et al. 2015).

Thermography and exercise under extreme heat conditions

In Figure 4, we can observe an example of a thermogram of an athlete immediately after training under heat stress conditions. His skin describes a typical pattern seen in scientific publications (Brito et al. 2020; Mi et al. 2019) and is usually related to immunosuppression. We call this thermal pattern "Dalmatian" because of its mottled appearance. In this article, we provide a detailed description of the characteristics and conditions that usually produce this phenomenon in response to cardiovascular fatigue.

Figure 4. Example of a Dalmatian pattern in a male athlete. The areas of hot spots, which allows us to identify this pattern. Property of ThermoHuman.

In general, this is also the usual pattern in conditions of extreme heat, especially when profuse sweating is present. The mechanisms that produce this phenomenon lack, for now, scientific evidence, although they can certainly be explained by the alteration of dermal blood flow during the heat dissipation process.

Conclusions

Exercise in extreme heat carries a series of risks for the health and performance of the athlete, among which heat stroke stands out. Rapid cooling strategies, such as immersion in cold water, are essential in this type of situation, both for prevention and treatment. Thermography makes it possible to assess the elevated temperature of the players, facilitating good practices in the heat adaptation process. Finally, during the days or weeks prior to the competition, and with the aim of ensuring correct training and recovery in heat, the metrics of coefficient of variation and smoothed coefficient of variation make it possible to monitor the evolution of the player's adaptation to extreme temperatures.

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