By Dr Stéphane Bermon
Why hot environments are problematic in sports?
Sport is by definition associated with physical activity involving the contraction of a large number of skeletal muscles. These muscle contractions repeated for several minutes or hours produce intense heat. The energy created being very close to that of a combustion engine; that is to say about 25%. The remainder 75% of the energy produced by skeletal muscle is dissipated as heat leaving 25% converted to mechanical energy. This heat generated by the muscular exercise must dissolve, to avoid triggering a progressive increase in core temperature (hyperthermia). Since human beings are homeostatic animals, their central temperature must remain between 36.5 and 37.5 °C. A temperature above 40 degrees Celsius represents a significant and immediate risk for the athlete. Fortunately, the mechanisms of regulation of body temperature are multiple and well developed in humans. Among these mechanisms, the main one is sweating. Sweating is water evaporation from the surface of the skin that dissipates a significant amount of heat into the surrounding atmosphere. It is essential to understand the lower the hygrometry (humidity of the air) of the ambient air the higher the sweating rate. Thus, sweating will be reduced or almost stopped if the subject performs intense/prolonged physical exercise in a very humid atmosphere, where the air is saturated with water. As a result, heat and particularly heat plus high humidity are real challenges for exercising individuals.
Exercising (and sweating) in the heat increases the risk of dehydration for all athletes and especially those who do not drink enough liquids. In very hot environments, exercising athletes may drink more than one litre of water or sports drink per hour and still develop mild dehydration. This level of dehydration should never exceed 2-3% of body weight which is 1.5 l to 2.0 l for a 70 kg individual. It is generally accepted that a dehydration level above 2% of body weight is associated with a decrease in physical performance whereas dehydration above 5% of bodyweight can be a significant health risk. Another risk when performing endurance activities in the heat is exercise-induced hyponatremia. Observed in unexperienced marathon runners, it is a medical condition that unfortunately kills dozens of runners every year. When exercising in the heat for several hours some individuals are over-drinking still water without any additional sodium. As thesweating process is at its maximal, sodium losses are also very high. If notcompensated by the drinks or food, the level of sodium in the blood slowly decreasesto a hazardous level called hyponatremia. This biological disturbance causessevere neurological symptoms and death if not rapidly corrected. One of the reasons why many sports drinks sold on the market now contain extra sodium.
“The best way to prepare for competing in the heat is to acclimatise and train in the heat.”
Are all sports affected?
Not all sports are affected by hot environments. Sports considered to beat risk are endurance sports that assume high levels of muscle contraction for several periods oftens of minutes to several hours. Also, if these endurance sports are practised outdoors, and the athletes are subjected to intense sun exposure, this constitutes an additional risk factor. Finally, outdoor sports for which athletes must wear protective clothing are also risky sports. Thus, one canconsider that sports like a marathon, race walking, football, tennis, triathlon, rugby and cycling are sports for which meteorological factors must absolutely be taken into account. In addition to potential risks for health, hot environments are also detrimental to performance. For instance, some researches showed that a marathon race is run 3 to 4% slower in temperatures above 25°C when compared to a cooler environment.
Are all athletes equally vulnerable to the heat?
No, not all athletes are exposed to the same risk of heat related illnesses. Children and women are considered the primary populations at risk. Children and adolescents have less adaptation to heat because their brain thermoregulatory centres are not fully matured. Women are also at greater risk because of higher subcutaneous fat, which hinders thermoregulation mechanisms. Moreover, in young women, the second phase of the menstrual cycle is associated with an increased production of progesterone which naturally increases their core temperature, thus favouring hyperthermia. Finally, athletes who are regularly exposed to hot environments are considered acclimated and can better tolerate the climate.
The next Olympic Games is in Tokyo during their summer. How high isthe risk of heat-related illnesses?
The next Olympic Summer Games is hosted in Tokyo in August 2020. The temperature records of the last 50 years in this city, at this time of the year, show likely temperatures fluctuating at 29°C and 35°C between 10am and 6pm. Also, the humidity level will be expected to be above 70% and the solar radiation maximal. Thus, all outdoor sporting events during these hours represent a real risk of hyperthermia, heat stroke and dehydration for athletes. The workforce present at the Olympic Games, as well as the spectators attending these outdoor competitions, are also exposed to these risks. This may be particularly true for elderly subjects, cardiac or obese individuals or patients treated with certain medications. The International Olympic Committee, in agreement with the Tokyo 2020 Organizing Committee and the leading international sports federations have decided on an action plan that will mitigate heat-related risks for athletes, workforce and spectators. As a part of this action plan, rescheduling of some sports competitions early in the morning or late
on the afternoon has been seriously taken in consideration by both the International Olympic
Committee and the Tokyo 2020 Organising Committee.Information campaigns before and during competitions will be organised. Many refreshments and shade points will also be proposed.
What are the mitigation strategies for the athletes competing at the Tokyo 2020 Olympic Games?
The best way to prepare for competing in the heat is to acclimatise and train in the heat. Achieved through repeated exercise-heat exposures that increase body core and skin temperatures, as well as inducing significant sweating. The number of days required to obtain optimal acclimatisation varies but most adaptations develop within 7-10 days, with 14 days being optimal. It is thus recommended to train in a similar environment to the one in which the competition will occur two weeks before competing in hot and humid conditions. If travelling in tropical countries is not possible, artificial acclimatisation (called acclimation) is achieved by mild training in hot environments like a sauna, hammam or simply a hot bath. Although the human body cannot store water, it is advised that athletes have a serious hydration plan prior, during and after training and competitions in the heat. There are other “funny” countermeasures which have proven their efficacy and safety to mitigate the effect of heat. Ingesting ice slurry 30 to 45 minutes prior a lasting exercise in the heat delays the onset of hyperthermia and is associated with a better performance in a hot environment. Similarly, wearing an ice vest, prior competition allows the core temperature not to increase during the warm-up and also delays the onset of hyperthermia. Immersion in an ice bath for heat stroke-affected athletes can also be used as a very effective treatment to save their lives when their core temperature reaches above 40 degrees Celsius.
Thousands of spectators will attend and enjoy the next Olympic Games in Tokyo. What advice would you suggest?
It is essential that these sports fans do not forget to wear a hat, sunglasses, use sunscreen and take much beverage with them. They should avoid standing in queues under the sun. Standing or seating in the shaded area should always be preferred, especially for children, elderly persons or individuals with chronic medical conditions. Last but not least, the Tokyo 2020 organisers will also display weather forecasts and actual conditions so the public can anticipate and take appropriate measures to prevent any heat-related symptoms or disease (sunburn, dehydration, heat stroke, collapse,…). However, this should not prevent sports fans to travel to Tokyo to enjoy the experience of the Games.
Dr Stéphane Bermon is a sport physician and exercise physiologist based in Monte Carlo
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