Heat acclimation in football players

This is a special post with regards to the FIFA World Cup 2014 as its different environmental conditions at each stadium (temperature and humidity) were fairly often debated in the press already.


Generally, Brazil’s average daily temperature during this time of the year is warmer than many other countries. Research from international matches in Qatar under hot environment significant favored the Qatari team showing the influence/advantage of heat accustomized teams (1 - see references below).


In sport science terms, getting accustomized to a hot (and/or humid) environment is called heat acclimation, which was a topic of interest for many years for exercise physiologists, especially for sporting events such as Tour de France or for military purposes.


What happens in heat and furthermore when exercising in heat?

Generally, the human body operates on a core temperature of 37 degrees Celsius. So (only) environmental heat (by itself already) will put a physiological stress on the body, which is therefore challenged to keep its temperature (mainly the core) at its original base temperature. Football matches played in hot environmental conditions can alter the core temperature and a core temperature of up to 40 degrees celsius were observed in footballers (10).


However it is not only the core, but also the central nervous system (i.e. brain) that needs to be prevented from a tremendous increase in temperature and possible catastrophic failure.

In order to maintain temperature the human body has different mechanisms: conduction, convection, radiation, evaporation.

Conduction – shows the ability to loose (or gain) heat through direct contact with a source of lower (of higher) temperature (such as touching a cold-pat or the stove).

Convection – is thermal heat transfer trough liquid or gas, in which the cooler liquid or gas takes over the warmer areas which have risen higher. Every air conditioning is doing that.

Radiation – is the heat transfer through infrared radiation. An example here would be the sun.

Evaporation – is the heat absorption by liquids which then becomes a gas. This is basically the ability of the humans body to maintain its temperature through sweating.  Another example is if you leave a warm cup of water around and it is evaporated after a while.

As it seems, evaporation seems to be the only possibility to release heat during the match, while, especially in half-time other options can be chosen. An example for half time cooling strategies would be cold and/or wet towels that the footballers will put on (parts) of their body.

However, it would be even more beneficial to NOT having a great(er) stress on the body in first place. As a result pre-game strategies also included experiments with cold showers (4), ice vests (7), cold towels (5) or ice-slushies ("cream" of crushed iced that was consumed by the athletes), which was thought to provide water replacement and also carbohydrates if needed. The conclusion from the authors in the studies was that transfer of knowledge from the laboratory to the field is difficult and the provided pre-cooling options did not provide any performance benefits.

As a result heat acclimation should be applied.


Physiological adaptations of heat acclimation (15)

Physiological strain - reduced         
Aerobic performance - Improved
 Core temperature – reduced
- Sweating – Improved
    •    onset of sweating – Earlier
    •    sweating sensitivity – Increased
    •    sweating capacity – increased
    •    ability to sustain – Improved
    •    distribution of sweat – Improved
- Skin temperature – Reduced
- Skin blood flow – Improved
- Metabolic rate – Lowered
- Muscle glycogen use – Reduced
  Heart rate – Decrease
- Stroke volume – Increased
- Cardiac reserve – Increased
- Blood pressure – Better defended
- Myocardial compliance – Increased
- Myocardial efficiency – Improved
- Fluid balance – Improved
- Thirst – Improved
- Electrolyte loss – Reduced
- Total body water – Increased
- Plasma volume – increased & better defended


However, pure heat acclimation cannot be replaced by physical training (2)

That means, the acclimation procedure (= sitting in the heat for example), will only improve the (passive) heat tolerance (whilst sitting) and the pure rise in core temperature is now enough to provide adjustments to the body when exercises. Generally, heat acclimation require repeated heat exposure over many days (7-14) (12) are and the adaptations are specific to the stimulus. Furthermore, exposure time for acclimation is about 100 min/day (see presentation below).


So how many days were chosen by the favored teams to win the WC?

Team Arrival First Game Days to acclimatize
Spain 8.6. 13.6. 5 days
Germany 7.6. 16.6. 9 days
England 8.6. 15.6. 7 days
France 9.6. 15.6. 6 days
Italy 6.6. 16.6. 10 days
Argentina 9.6. 16.6. 7 days
Netherlands 6.6. 13.6. 7 days


It seems that the teams chose between 5 to 10 days to acclimatize for the first game.

However, as the teams will also train, it could be possible that the training will add/benefit to the acclimation. That means (and in combination of earlier statements) the heat acclimation has to be performed/maximized with/through (football specific) activities.

For example it was reported that the Italian team did some cycling sessions inside a sauna to replicate the environmental conditions (3).


What does the research suggests for training/competing in the heat?

It seems that heat acclimation is a very individual process (and there are slow and quick responders) and acclimation can be successful within 7 days in semi-professional footballers, despite a significant decrease in total distance covered in the hot environment (13). A study showed that the players with the greatest changes in haematological adaptations were able to maintain performance in comparison with the football match under normal temperature (14) and the fitter the players the quicker you acclimatize.

Match running performance was impaired by 7% (10) and up to 11% (-6.0 ± 5.8) (14), high-intensity running by 26% (10) -30% (16.4 ±21.5) (14). Changes in hematocrit were correlated with total running during the came (r = -0.75). However (and interestingly), the success rate of passing and peak sprinting speed was greater in hot conditions (10).

Due to the physiological strain it seems obvious that players will pace themselves (6) or just cover lower distance (11) if not acclimatized.

Obviously, fluid replacement plays a crucial role (16), as fluid loss can cumulate up to 5 l per match and it was suggested that performance decrement in aerobic endurance (9, 12), high-intensity running and repeated sprint (9) was correlated with the grade of dehydration.

A recent study suggest that after heat acclimatization, the improvements of the players will carry over to the cold environment (8).



Image taken from: http://www.fifa.com/worldcup/destination/
Image taken from: http://www.fifa.com/worldcup/destination/

Heat acclimation needs to be very specific and in my opinion it would be good to have tried acclimation strategies for the WC.

It might be possible that intensity of the matches might improve throughout the tournament, which could be due to the heat acclimation but also (of course) the importance of the knock out stages. Additionally, it seems plausible to have greater affect of weather conditions for games played in stadiums located in the north of Brazil (for example Fortaleza 29 degress celsius and 74% humidity as of today), as greater heat and/or humidity is present in this part of the country, compared to matches in the more southern part of the country (for example Porto Allegre 22 degrees celsius and 69% humidity as of today).

However, this is speculative and I am very curious to see if the environmental conditions will affect the performance of the teams. Furthermore, it also seems questionable if the Brazilian players are acclimatized, as many players are playing in Europe. Therefore it will be very interesting to see if any European teams can surprise (mainly the southern american teams).

A picture on the Telegraph from the UK sums up nicely my entire idea/any predictions so far.



If you are interested in more heat acclimation stuff, I suggest watching the youtube videos from the according conference at Aspetar from March 2014.



1. Brocherie F, Girard O, Farooq A, Millet GP. Influence of environmental temperature on home advantage

in Qatari international soccer matches. In: D.M. Peters and P. Donohue, Editors. Performance analysis of Sport IX: Routledge, 2013, pp 39-44.

2. Cohen JS, Gisolfi CV. Effects of interval training on work-heat tolerance of young women. Med Sci

Sports Exerc. 14(1): 46-52, 1982.

3. Dampf A Smart World Cup teams tap science to beat Brazil's heat. 2014.

4. Drust B, Cable NT, Reilly T. Investigation of the effects of the pre-cooling on the physiological responses

to soccer-specific intermittent exercise. Eur J Appl Physiol. 81(1-2): 11-7, 2000.

5. Duffield R, Coutts A, McCall A, Burgess D. Pre-cooling for football training and competition in hot and

humid conditions. European Journal of Sport Science. 2011.

6. Duffield R, Coutts AJ, Quinn J. Core temperature responses and match running performance during

intermittent-sprint exercise competition in warm conditions. J Strength Cond Res. 23(4): 1238-44, 2009.

7. Duffield R, Dawson B, Bishop D, Fitzsimons M, Lawrence S. Effect of wearing an ice cooling jacket on

repeat sprint performance in warm/humid conditions. Br J Sports Med. 37(2): 164-169, 2003.

8. Lorenzo S, Halliwill JR, Sawka MN, Minson CT. Heat acclimation improves exercise performance. J Appl

Physiol (1985). 109(4): 1140-7, 2010.

9. Mohr M, Mujika I, Santisteban J, et al. Examination of fatigue development in elite soccer in a hot

environment: a multi-experimental approach. Scandinavian Journal of Medicine and Science in Sports. 20 Suppl 3: 125-32, 2010.

10. Mohr M, Nybo L, Grantham J, Racinais S. Physiological responses and physical performance during

football in the heat. PLoS One. 7(6): e39202, 2012.

11. Özgünen KT, Kurdak SS, Maughan RJ, et al. Effect of hot environmental conditions on physical activity

patterns and temperature response of football player. Scandinavian Journal of Medicine and Science in Sports. 20(Suppl 3): 140-147, 2010.

12. Periard J, Racinais S Adjustments in football performance under heat stress. 2013.

13. Racinais S, Mohr M, Buchheit M, Voss S, Nybo L. Heat acclimatization in semi-professional soccer

players. The Journal of the Federation of American Societies for Experimental Biology. 26: 1084.7, 2012.

14. Racinais S, Mohr M, Buchheit M, et al. Individual responses to short-term heat acclimatisation as

predictors of football performance in a hot, dry environment. Br J Sports Med. 46(11): 810-5, 2012.

15. Sawka MN, Wenger B, Pandolf KB. Thermoregulatory Responses to Acute Exercise‐Heat Stress and

Heat Acclimation. Handbook of Physiology, Environmental Physiology: Wiley-Blackwell, 1996.

16. Shirreffs SM, Aragon-Vargas LF, Chamorro M, Maughan RJ, Serratosa L, Zachwieja JJ. The sweating

response of elite professional soccer players to training in the heat. Int J Sports Med. 26(2): 90-5, 2005.

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