Thursday, May 2, 2013

Playing Matches in the Heat: Dehydration and Performance

Often matches are played in hot, humid conditions. In this environment, the body attempts to cool itself by increasing the sweat rate. Unfortunately, the fluid lost through sweat can lead to dehydration. Laboratory research has shown that even mild dehydration can impact physical performance, reducing strength, power and endurance. Researchers from the United Kingdom and Denmark approached the question of heat, dehydration and performance in a different manner. They took their experiment to the pitch and asked if competing in the heat influences post-match physical performance. Their results show that playing elite, competitive matches in a hot environment adversely affect explosive performance and that the change in performance may be linked to dehydration.


The data were collected from players participating in six UEFA Champion’s League matches. Three of the matches were played at home in a relatively cool environment (12°C or 54°F) and three were played away in a hot environment (30°C or 86°F). Before and immediately after the matches, each player’s vertical jump height was recorded. Pre-match sleep patterns, diet and hydration were consistent for both the home and away matches.

Prior to the matches, the average jump height was 40.3 cm. After playing in the hot climates, height was reduced by 6% (2.4 cm). After playing in the cooler environment, jump height was reduced by less than 2% (0.7 cm).

The researchers also found that the hot environment caused a 3.1% loss of body weight compared to a loss of 1.7% in the cool climate. As expected, sweat rate in the heat was almost twice that of the cooler matches. Sweat rate and body weight lost was correlated with the post-match decline in jump height. That is, higher sweat rates and greater fluid losses translated into greater declines in explosive power.

It should be pointed out that it is possible that travel and playing away matches could have affected the jump height scores. Despite consistent pre-match sleep, diet and hydration, travel can affect performance. Also, the conditions of each match (home crowd, score line, etc) can affect effort, movements and the amount of fatigue encountered. However, given the stark differences in environmental temperature and the correlation between jump height and sweat loss, it is very likely that heat and dehydration played a key role in the outcome of the study.

Laboratory research shows that performance can be affected with as little as 1-2% body weight lost through sweating. Certainly weight losses of 3% or more have a great impact on performance. In this study, players competing in the hot environment lost an average of 3.1% of their body weight over the 90 minute match. In fact, four of the 19 players lost more than 4%. So it is not surprising that vertical jump height was affected after playing in the heat. What is unique about this study is that heat and dehydration encountered during a match seems to affect performance in a manner similar to that seen in controlled laboratory studies.

Because of the nature of the study, there is no way to tell if performance on the field, during the match was affected by the heat and dehydration. This is because vertical jump was measured before and after the match, not during. In the course of competition, Players may be able to overcome the negative effects that fatigue and dehydration have on performance. However, this study does emphasize the importance of dehydration for peak match performance, especially near the end of the match. A 6% decline in vertical jump or explosive power may seem small but it could mean the difference between winning a header and conceding a goal.

The study also stresses the need to be well-hydrated before kick-off and to drink, when possible, during the match. To avoid dehydration, follow these simple steps:
  • Drink plenty of fluids during the days leading up to the match.
  • Drink fluids whenever possible during the match, at stoppages and at halftime.
  • Begin re-hydrating soon after the match (and training), replacing ~1.5 times the amount of fluid lost within 24 hours.
  • Many sports drinks provide fluids along with minerals lost through sweat as well as carbohydrates.
The bottom line is that by avoiding dehydration during a match, players are likely to perform better, particularly near the end of the game.

Lastly, dehydration can result in severe health consequences ranging from mild headache to heat stroke. If a player is suspected of suffering from dehydration, he/she should be immediately removed from competition or training and cared for by a trained health care professional.

Reference

Nohr M, Krustrup P (2013) Heat stress impairs repeated jump ability after competitive elite soccer games, Journal of Strength and Conditioning Research, 27: 683-689.

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