There is no doubt that climate change is affecting global temperatures. The last decade, from 2011 to 2020, has been the warmest in the entire history of record (142 years), within a persistent long-term trend (National Oceanic and Atmospheric Administration, 2020; World Meteorological Organization, 2020). In 2020, the average global temperature was 1.2 °C above pre-industrial levels (World Meteorological Organization, 2020) and is projected to increase by 1.5 °C between 2030 and 2052 (Masson-Delmotte et al. 2018). Looking ahead to the Qatar 2022 World Cup, the fact of knowing and monitoring the state of heat adaptation in players is essential because performance will directly depend on it, as we saw in the previous article.
Heat acclimation is the process of exposing an individual to repeated heat stress for approximately 7 to 14 days, intending to increase whole-body temperature and induce profuse sweating. In contrast, heat acclimatization occurs naturally due to seasonal changes or travel from cold to warm places (Armstrong & Maresh, 1991).
An interesting review on heat stress in sports (Périard et al. 2021) points out that controlled, gradual and medium-term exposure to heat contributes to the generation of different adaptations. Among them, we can highlight cardiovascular adaptations, which can be seen in Figure 1 and include:
Figure 1. Cardiovascular and performance adaptations as a consequence of exercise under heat stress. Extracted from Périard et al (2021).
Controlled hyperthermia training seeks to achieve an elevated target core temperature which allows for faster and more complete heat adaptation relative to traditional heat acclimatization regimens during exercise at a constant work rate (Périard et al. 2015).
In addition, inducing heat acclimatization outdoors in a natural field can provide more specific adaptations based on direct exposure to competition conditions (Périard et al. 2015), hence the general recommendation to travel to the hot country 14 days prior to championships. ThermoHuman software can monitor and control heat acclimatization processes by measuring the coefficient of variation, thus controlling the variation of body temperature until the measurement is balanced. An example is shown in Figure X.
To conclude, we would like to mention that a very promising area of research in human physiology is the so-called heat science. Heat exposure seems to have the potential to contribute to the treatment of cardiovascular diseases, particularly in aging and at-risk populations, for whom exercise is not recommended (Cheng et al. 2019).
As was already the case in the 2014 World Cup in Brazil, where of the 64 matches, 28 were played in low environmental stress, 20 in moderate stress and 16 in high environmental stress (Nassis et al. 2015). The Qatar 2022 competition, is expected to have very similar conditions. Although the total distance covered, top speed, number of passes, and number of goals scored and cards were similar in all conditions, there was a clear loss in performance in other parameters that worsened under heat stress:
A very interesting study (Mohr et al. 2012) tested the influence of temperature on several performance factors by comparing pre and post-game results in two matches, one in extreme heat conditions (43 ºC) and the other in average temperatures (21 ºC). Average heart rate, plasma lactate concentration, body weight loss and post-game sprint performance were similar between the two conditions.
However, under heat stress:
A multitude of studies (Benjamin et al. 2021; Coker et al. 2020; Racinais et al. 2014; Mohr et al. 2013; Gray et al. 2010; Duffield et al. 2009; Morris et al. 2000) show similar results in soccer, and other intermittent sports, pointing to the body's inability to dissipate heat in extreme conditions as a key factor in the reduction of physiological performance.
We highlight the study by Chodor et al. (2021), in which they recreated the estimation of the environmental conditions that there will be in Qatar on the dates of the FIFA World Cup compared to intermediate conditions (28.5 vs. 20.5 ºC). They were intended to understand the extent to which peak athletic performance is compromised. Higher peak power, shorter time to peak power, higher power loss between repetitions, and higher pO2 concentration were found at the estimated competition temperature. In summary, at higher temperatures, a greater maximum effort capacity was achieved, but with a more pronounced reduction in performance.
In order to reduce the negative consequences of extreme heat, there are some evidence-based recommendations (Racinais et al. 2015), such as external cooling methods (application of iced garments, towels, water immersion or ventilation) and internal cooling methods (ingestion of cold liquids or ice in suspension). In fact, simply adding a cold towel around the neck appears to significantly reduce thermoregulatory stress and heat perception, as well as increase athletic performance (Misailidi et al. 2021). In addition, pre-cooling may benefit performance in hot environments (Racinais et al. 2015).
As seen in previous sections, the coefficient of variation can be a relevant indicator to identify the body's response to heat. In this regard, identifying what has been the thermal response after the competition allows us to choose the best strategies for recovery based on an individualized thermal profile.
Despite the lack of evidence in this matter, our practical experience has shown us that soccer players experience different thermal responses depending on the physiological demand to which they have been subjected.
In Figure 2, we can observe the theoretical framework and paradigm of recovery as a function of the type of fatigue. If there is metabolic fatigue or if it is structural damage, the body's response, and therefore the recovery strategy, will be the opposite. In this article, we explain this process in detail.
Figure 2. A practical framework for improving recovery in athletes by Thorpe (2021).
In a soccer team, we frequently find global thermal responses and behaviors in players 24 and 48h after a match (figure 3). Thus, depending on the origin of the fatigue, we can find three tendencies:
Figure 3. Example of the thermal response of a professional soccer team on the day after a match (MD+1). We can distinguish three main trends in the players analyzed.
Thermography makes it possible to monitor acclimatization to heat stress environments by tracking and individualizing thermal profiles. The coefficient of variation provides us with information on how the temperature acquires a degree of equilibrium as the body reaches acclimatization to the hot environment.
In addition, in environments such as those that occurred in Brazil 2014 or will occur in Qatar 2022, monitoring the post-match thermal response can help identify what type of thermal response players are having on an individualized basis, in order to prescribe the best recovery strategies. In this way, we will be optimizing training processes and fatigue management in such congested schedules.
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ThermoHuman has had the support of the Funds of the European Union and the Community of Madrid through the Operational Programme on Youth Employment. Likewise, ThermoHuman within the framework of the Export Initiation Program of ICEX NEXT, had the support of ICEX and the co-financing of the European Regional Development Fund (ERDF).
Thermohuman has received funding from the Centre for the Development of Industrial Technology (CDTI), in participation with the European Regional Development Fund (ERDF), for the R+D activities involved in creating a new tool, based on thermography, for the prediction and prevention of rheumatoid arthritis. See project detail.
