• Heat acclimatisation
• olympics
• heat stress
• training
• testing

The 2020 Tokyo Olympics will be held from 24 July to 9 August, in hot and humid environments, in the world’s largest metropolitan area. Two years to the day before the 2020 Olympics, temperatures surpassed 41°C with over 65 heat-related deaths recorded in a single week. Protecting the health and performance of the athlete under these conditions is a shared responsibility among the International Olympic Committee (IOC), the local organiser (LOC), the International Federations (IFs), the National Olympic Committees (NOCs), the national federation medical staff, the coaching staff and the athletes themselves. Unfortunately, these various stake holders do not always have the appropriate information to adopt the necessary preventive countermeasures. Following the motto ‘By failing to prepare you are preparing to fail’ (Benjamin Franklin, 1706–1790), a series of editorials will be published between now and the Olympics with clear information for practitioners on how to prepare.

First of all, endurance and team sports athletes should be aware that heat stress dramatically decreases performance. Fortunately, the acute effects of heat stress can be progressively attenuated by repeated training in the heat either in natural (ie, heat acclimatisation) or artificial (ie, heat acclimation) environments.1 It is important for medical personal, coaches and athletes to consider that heat acclimatisation is the most important countermeasure one can adopt to protect the health and performance of athletes.2 The general consensus is that athletes should train 60–90 min per day in the heat for ~2 weeks before the competition.2 However, although human physiology is common, interindividual differences to exposure largely influence the time course and magnitude of adaptation to heat acclimatisation.

The notion of individualised responses to heat exposure is not new. For example, Havenith and van Middendorp3 nicely showed 30 years ago that core temperature at rest or at a given relative exercise intensity depended on both anthropometric (ie, body fat and surface area: to mass ratio) and physiological (ie, VO_2max and sweat rate) parameters. Moreover, individual physiological characteristics (sweat rate and VO_2max) also influenced the individual heart rate response in the heat. More recently, genetic attributes, independent of anthropometric and fitness characteristics have been shown to account for interindividual differences when exercising in the heat, with smaller increases in temperature and heart rate evident in individuals with the ACE I+ polymorphism compared with DD polymorphism.4 Contrary to popular belief, classic studies showed that sex had no influence on the thermoregulatory responses once VO_2max and anthropometric data were accounted for.3 Even the menstrual cycle and oral contraceptive may not affect postexercise heat loss.5

Interindividual differences are also evident in the adaptive responses to repeated heat exposure, that is, heat acclimatisation. For example, in footballers undergoing identical acclimatisation procedures, large decrements in physical capacity were still evident after 1 week of acclimatisation in some players, whereas others were able to achieve similar match activity levels in the heat comparable with temperate conditions.6 Importantly, such differences should not be interpreted as responders versus non-responders, as humans, in general, adapt to heat,2 but rather as slow versus fast responders.6 In that context, it is particularly important for practitioners to determine the timeline involved in preparing each of their athletes (figure 1). No anthropometric or physiological measures can, however, provide this information.6 7 Even physical tests in temperate environments cannot predict what will happen when exercising in the heat.8 In other words, the only way to determine how an athlete will respond in the heat is to test this athlete in the heat, and the only way to determine how the athlete will adapt is to perform an acclimatisation procedure. Measuring the changes in haematocrit levels after 1 week of heat acclimatisation has been shown to predict the benefit of short-term acclimatisation on physical capacity in various sports.6 9 10 However, this correlation disappeared after a week10 due to the multitude of other adaptations that are associated with an acclimated phenotype,2 and the fact that these various adaptations are partly independent of each other, with fast responders on one marker of acclimation not necessarily demonstrating a similar rate of adaptation on others.7

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Figure 1

Individual responses of two different athletes to a 3-week heat acclimatisation period: athlete A obtained most of the adaptation in 1 week; athlete B kept improving for 3 weeks (unpublished data).

In summary, it is imperative to heat acclimatise before any prolonged competition in the heat to protect the health and performance of the athlete. Suboptimal acclimatisation is still better than no acclimatisation.2 However, it is important to consider that some athletes may require 7–10 days only, but others >2 weeks, especially for female athletes.11 Therefore, we strongly recommend practitioners to organise an acclimatisation camp several months before Tokyo 2020 to determine the time course of adaptations and the individual needs of their athletes based on their physiological responses to exercising in the heat.