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In his new blog, Dr. Vincent Gouttebarge discusses the influence of temperature on health and football performance.

Dr. Vincent Gouttebarge, PhD:

Since the Bosman ruling in 1995, professional football players have been exponentially on the move across countries and continents. Even more, this international mobility has been encouraged by the recent globalisation in professional football: top clubs have been focusing strongly on far away lucrative markets, while national and international competition calendars have been integrating with one another. Consequently, professional players are exposed within and between seasons to different environmental conditions. Especially, consecutive weeks of training and competition in hot conditions are very common in most continents. Depending on the abilities of a player's body to cope, heat conditions might impact on the quality of the game itself but also on the player's health. Therefore, an awareness of the risks of playing in hot conditions and potential protective measures is essential. 

Body response to hot conditions
Because of its normal regulation mechanisms, the human body maintains a constant core temperature that usually ranges from 36.1 to 37.8 °C, allowing only relatively small variations. When playing in hot weather (in combination with humidity of the air), heat production is increased and players have to deal with an increased internal body temperature (>40 °C ) and accompanying excessive loss of water (and related substances such as minerals) through sweating (Wilmore 2007). 

The transfer of heat to the environment (outside the body) is allowed by the blood stream, moving the heat from deep in the body to the skin surfaces. Once near to the skin surface, heat is transferred outside the body. During exercise, transfer of the heat out of the body is principally allowed by sweating (evaporation), leading to a significant water loss. When exercising heavily in hot conditions, sweat loss peaks to 3.5 litres per hour, a rate that cannot be maintained for more than a few hours (Wilmore 2007). By contrast, in normal temperature condition (10 °C), football players lose on average 2 litres of fluid during a 90-minute game. Because of the intermittent and intensive nature of football, there is evidence to suggest that the increase in core temperature in players may be greater than in athletes involved in continuous-output sports (Wilmore 2007). 

Even if sweating allows a large heat loss, core temperature will increase during training or matches in hot conditions, as not all of the heat produced can be transferred outside the body. When normal regulation mechanisms cannot remove i.e. control the increased heat production (that means that water loss through sweating is not compensated and core temperature is too high), performance will be affected and heat-stress disorders may occur. 

Heat-stress disorders
Exercising in hot weather, in combination with the inability of the body to remove the heat produced and maintain its core temperature, may lead to serious heat-stress. Players who train and compete in hot weather conditions are likely to suffer from physical and mental symptoms, organ tissue damage, and at times even death (Wilmore 2007). During the latest half-marathon in Tel Aviv, one runner died because of the hot condition (32 °C), while in the United States, heat-stress disorders are the third leading cause of death in high school athletes. Among heat-stress disorders, a distinction can be made between heat cramp, heat exhaustion, and heat stroke (Wilmore 2007).

Heat cramp: least serious of the heat-stress disorders, characterised by (severe) muscle cramps involving the muscles most heavily used during exercise (calf muscles in runners and football players; arm muscles in tennis players) 

Heat exhaustion: characterised by extreme fatigue, breathlessness, dizziness, vomiting, headache, fainting, higher heart rate, and lack of muscle co-ordination

Heat stroke: most serious of the heat-stress disorders, characterised by a rise in core temperature to more than 40 °C, cessation of sweating, lack of muscle control, altered mental state (confusion), and unconsciousness.

Influence on the game
Next to the potential occurrence of heat-stress conditions, playing football in heat conditions is likely to have impact on the football game itself. Football is a complex sport placing many physical demands on the players. While excellent endurance is a mandatory condition to reach professional level, a match outcome might be especially reliant on the (physical) ability of players to repeat high intensity sprints throughout a match. Consequently, when professional football is played in hot conditions, the capacity of players’ bodies to cope with this adverse environment has essential influences on the physical performance of players and thus on the game itself.

In 2010, Özgünen and colleagues observed the physical activity patterns and temperature response of Turkish semi-professional football players during a match played in hot conditions (average ambient temperature of 36 °C). Being at 36.7 °C after the warm-up, the average core temperature of the players reached 39.6 °C (individual values as high as 40.2 °C) during the game. Due to the hpt conditions and related fatigue occurrence, the average total distance covered by the players in the second half was significantly less than the distance covered in the first half (3761 m vs. 4301 m). Identical findings were found by Mohr and colleagues (2010) during a match at 31 °C, conditions inducing increased fatigue (repeated sprint and jump performance) at the end of the game.

While more scientific studies are needed to explore body response to hot conditions among professional football players, it seems clear that hot conditions increase fatigue occurrence in the later stages of a game. This may be even more of an issue when training and playing on artificial turf. Anecdotic measurement on a summer day showed that environmental temperature at ankle level on artificial turf was up to 50 °C, which may increase the risk of heat stress-related health problems and may have negative influences on the quality of the game. 

Acclimatisation, active cooling and rehydratation strategies
With regard to the aforementioned consequences of hot conditions, it seems imperative to follow some relevant strategies in order to limit the body response, prevent heat-stress disorders, and to minimize influences on the football game (Duffield 2012; Garrett 2012; Maughan 2010). 

A period of acclimatisation is necessary in order to adapt to conditions of heat. For most individuals, about 7 to 14 days of acclimatisation may be sufficient. This period of acclimatisation is even more important when subsequent matches are played in hot conditions such as during a World cup.
Reducing body temperature before a match or at half-time (immersion in cold water or use of a cold vest) has been shown to have some positive effects. 

Replacing the water loss (rehydration) should be a major concern when playing in hot conditions. The best method is probably to have drinks rich in carbohydrates and sodium available continuously so that the player drinks whenever he/she wants but this is not applicable during a football match. Extra water breaks during each half have long been introduced in football in hot conditions in some countries.

Medical guidelines
Even if no documented cases of heat stroke or death due to hot conditions have been reported among professional football players, the FIFA has tried to establish some guidelines to prevent the occurrence of heat-stress disorders during international competitions. According to these guidelines, all decisions with regard to the health and safety of players are based on the ambient temperature as follows:

  • moderate risk for ambient temperature (in conjunction with air humidity) between 25 and 31.9 °C (77–89.4 °F);
  • high risk for ambient temperature (in conjunction with air humidity) between 32–38 °C (89.6–100 °F);
  • extreme risk for ambient temperature (in conjunction with air humidity) >38 °C (>100 °F).

Despites these guidelines, the football final of the Beijing Olympic Games 2008 (Nigeria - Argentina) was played at noon with an ambient temperature greater than 32 °C. It is worth noting that the International Rugby Board Heat Guideline and the American College of Sports Medicine apply a similar temperature limit of around 30 °C (also taking air humidity into consideration) for training and competition conditions. 

Key points

  • Playing football in hot conditions increases the body core temperature from around 37 °C to more than 40 °C, which is responsible for an excessive loss of water by sweating.
  • Exercising in hot weather, in combination with the inability of the body to maintain its core temperature, may lead to heat-stress related disorders such as heat cramp, heat exhaustion and heatstroke.
  • Playing a football match in hot conditions has a negative influence on the game as fatigue in players occurs earlier in the second half, inducing a decrease of the distance covered and both sprint and jump performances.
  • Acclimatisation (7 to 14 days), body temperature reduction and sufficient fluid intake are essential to limit the consequences of playing football in hot conditions.
  • Training and competition activities at an ambient temperature in excess of 30 °C should be avoided.

 

Former professional football player Dr. Vincent Gouttebarge is senior researcher at the Coronel Institute of Occupational Health from the Academic Medical Centre in Amsterdam (the Netherlands), and is co-owner of Vintta, a research and consultancy unit for sports health.