In team sports, competition and training can produce a significant loss of fluids and significant consumption of muscle glycogen, as a result of energy expenditure and the consequent production of metabolic heat. In the majority of team sports in the field, average transpiration rates of 800–1000ml/h can be observed, but losses that reach 150 or 200% of these indices can also be produced in humid climatic conditions, high temperatures or in certain players.1–5 The fluid intakes reported in the investigations range from 200 to 1400ml/h and can reach from 10%6 to almost 90% in losses due to sweating.7 In general studies report accumulated deficits during a football match ranging from 1.5 to 2% of body mass; some players could even exceed those levels (i.e., 4–5%), while other players can maintain a better hydration state. Even though the average level of accumulated fluid deficit during a session seems moderate (<2% body mass), the data indicate that players begin training or competition with a certain level of dehydration.2,8,9 Under these circumstances, it is likely that even a slight to moderate fluid deficit produced during the match may affect performance.

Even though the opportunities to drink liquids are not limited, many team sports players do not consume liquid in the amount necessary to equate sweat losses.1,3–5,8,9 The options and availability of liquids play an important role. In the case of soccer players, they usually lose 2–2.5kg of body mass when they are playing in a hot environment (i.e., >30°C)1,6 due to the limited opportunity to ingest liquids during a match soccer, only in half-time rest and breaks in the game.10 In addition, it was determined that soccer players drank less during a session when the liquid was at room temperature than when the liquid temperature was cold.2 The ambient temperature influences the intake of liquids regardless of the actual needs of drinking. During training in the same group of soccer players in summer and in winter, the balance of liquids was compared and little difference was found in fluid deficits; although the transpiration rates were lower with cold temperatures, the players also decreased their fluid intake.1

The progressive dehydration experienced by soccer players could cause deterioration in the execution of the game because dehydration induces fatigue which in turn could hinder motor11 and cognitive12 functioning. In addition to affecting muscle work, the negative effect of the fluid deficit on mental state and concentration is added, which is an important factor in determining the result of matches.5 It has been reported that a large dehydration (3.8% body mass, using diuretics), despite reducing the maximum isometric force, increases the vertical height jump probably because it increases the strength of the leg in proportion to body mass.13 Taking a leap to nod is a highly demanded task during a football game. A large dehydration could result in a large jump in height, although the effects of moderate dehydration (<3% of body mass, more common in football) on the vertical height jump have not been reported. Dehydration alone (4% loss of body mass) does not affect the strength of the extensor muscle of the knee measured during maximal isometric contraction.14 This suggests that dehydration may not affect the execution of a penalty kick in football which greatly depends on the strength of the knee extensor. The literature is not conclusive in terms of the effects of fatigue on motor control, although there are studies suggesting that the acuity of the movement is diminished due to muscle fatigue.15 With decreased central controls, individuals tend to decrease the speed at which they perform a task to maintain the same level of accuracy.16 It is not clear yet whether exercise in a hot environment that induces dehydration will diminish central control and whether it will affect acuity or speed during a task such as the finishing of a penalty kick in a football game.

This study was aimed at determining whether the hydration status of a professional soccer player affects their physical, technical and tactical condition. Specifically, the effects of hydration status have been examined in physical tasks (vertical jump) and motor (penalty kick), the latter initiated by auditory stimuli in comparison to when they are motivated by a stimulus that involves the recognition of a visual signal and the decision making before the auction. The state of hydration was modified through a situation that produced dehydration (i.e., training) and in which the soccer players were rehydrated with water ad libitum or according to nutritional recommendations.

The urine sample collected before the training indicated that the players arrived at the training session with a similar state of hydration (Table 1). On the day that the players drank water ad libitum, the dehydration was significantly greater than on the day on which they followed some hydration guidelines (Table 1). However, the total fluid intake did not have significant differences between sessions (p=0.79). On the other hand, the calculation of the sweating rate of each of the sessions resulted in a significant difference between both, with more sweating being the day of water consumption ad libitum vs. the day of water recommendation (Table 1).

In the case of the evaluation of vital signs, was observed that the heart rate increased significantly during both sessions. The values obtained prior to the training sessions were similar, and in both sessions the heart rate values showed significant changes when evaluating post-training values (Table 2). However, the day that the players hydrated following the recommendations given, the heart rate increased significantly when compared to the session where there was hydration ad libitum. There was no treatment×time interaction in the heart rate variable (p=0.39).

On the other hand, the tympanic temperature only increased during training in the case of the session in which hydration recommendations were given (p<0.01), while the temperature remained stable in the session with hydration ad libitum (p=0.07). There were no differences between the previous measurements of both sessions, unlike the values of tympanic temperature obtained after the training sessions, where the players presented higher temperature after training with hydration recommendations compared to the session with hydration ad libitum (p<0.01). In this case there was interaction between treatment and time (p<0.01) (Table 2).

Jumping height was similar before training in both sessions (p=0.15). When the players drank ad libitum, the jump height did not change after training (p=0.15). However, when the players followed the hydration guidelines, the post-training jump height was higher than that of the start (p<0.01) and also higher than that of the ad libitum water consumption situation (p=0.04). There was no interaction in the treatment×time relationship in the jump height variable type SJ (p=0.42, Fig. 2).

Figure 2.

Changes in the height of the SJ jump depending on the type of hydration considered.

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Table 3 shows the accuracy of goalpost shooting. As indicated by the data, there were no changes in the experimental tests with auditory signal neither in the comparison between pre vs. post-training nor in the comparison between hydration ad libitum and recommended hydration. On the other hand, in the case of shots on goalost with a visual signal, there were no changes in the experimental tests in the pre vs. post-training comparison, nor in the comparison between both pre-training sessions and both post-training sessions. However, there was a significant difference in the treatment/time ratio in hits from shots to goalpost with visual signal (p=0.01). It should be noted that the scale of perception of effort (Borg, 1975) did not present differences between both training sessions (15.4±1.4 points vs. 15.5±0.7 points; p=0.79).

The main findings of this study report that the dehydration achieved and the rate of sweating was greater when the players drank water ad libitum compared to the recommended rehydration. However, an increase in the post-training jump height was observed and a decrease in the number of errors in the shots on goal when the players followed the hydration guidelines.

In both sessions the players came to training with a similar state of hydration. On the other hand, the calculation of the sweating rate of each of the sessions gave a significant difference between both, with more sweating being the day of water consumption ad libitum. In the case of the evaluation of vital signs, was observed that the heart rate increased significantly during both sessions, especially in the one that hydrated following the recommendations given. Tympanic temperature had significant differences only in the case of the session in which hydration recommendations were given, in this case if there was interaction between treatment and time. It is known that the rate of sweating can vary depending on environmental factors such as temperature, relative humidity and wind speed, or other factors such as exercise intensity, physical condition, acclimatization to heat or the use of sportswear that they favor the exhalation of sweat through the skin.9 So it is important to note that during this investigation the environmental temperature at the level of the field of play was higher in the second session, that is, the day where hydration recommendations were given, however, the humidity was higher on the day of water intake ad libitum. On the other hand,21 in their investigation, after 90min of continuous exercise (with a temperature of 23°C and 23% of relative humidity) without hydration, the men reported a loss of body mass slightly higher than that of the women, while maintaining a personalized hydration protocol, said change was not manifested and the physiological responses were similar in both sexes. In addition, they found that in that case the degree of dehydration did not alter the physical performance, but the physiological parameters (temperature, heart rate and blood pressure) regardless of sex.21

Loss of more than 2% of body weight after exercise may affect athletic performance due to decreased body fluid.20,22,23 To generate a loss of 2% of weight due to dehydration, brings hyperthermia as a consequence, which increases the work of the cardiovascular system, generating an increase in the perception of effort, reduction of blood flow and alteration in the metabolism in the skeletal muscles during the exercise.24–26 On the contrary,16 indicate that the intake of water, by itself, attenuates the decrease in neuromuscular force produced by exercise of moderate intensity in a hot environment; more if 6% carbohydrates are added to this liquid, the effect is even greater on the neuromuscular force.

The fact of avoiding the dehydration of the players improved the height jump and reduced the error rate of the penitentiaries with visual signal. When the players followed the hydration guidelines, the post-training jump height was higher than the start and also higher than the ad libitum water consumption situation. Regarding the accuracy of the shot at goalpost, there were no changes in the experimental tests with auditory signal, but in the case of shots on goalpost with visual signal there were no changes in the experimental tests with relation pre-training vs. post-training, however, if there was a significant difference in the treatment×time relationship. It should be noted that the scale of perception of effort19 did not present differences between the two training sessions. These data together could indicate that a moderate dehydration could affect the muscular strength of the leg and reduce the motor precision during a simulated soccer penalty kick. It should be noted that the greater the level of dehydration, the greater the physiological tension and the decrease in performance in aerobic exercise.23

The execution of a soccer skill (dribbling) deteriorates when adult soccer players become dehydrated when exercising in a thermoneutral environment27 while the perceived strength increases in comparison to the rehydration test. As in the results obtained in this study, the aforementioned investigations also refer to the increase in the error rate. The accuracy of the penalty kick was reduced by fatigue induced by dehydration, especially when introducing the decision-making variable. This result could be interpreted, taking into account that the visual signal produced longer response times as opposed to the auditory signal, which seems to have given time for the accuracy to be restored despite the hydration condition. The delay in this response could be explained by the speed-precision exchange principle.28 If the fatigue induced by dehydration decreases the control of the motor units, whether speed or precision, the task performed will be affected. By introducing two cognitive components in the penalty as initiators for the response of the shot and taking into account that both signals, auditory signal through the propagation of sound and light propagation for the visual signal through color cards, were much more fast that the response time observed could suggest that the nature of the signal was not a factor that affected the motor response time for the shot. Severe dehydration increased the height of the vertical jump by 7% probably due to a greater reduction in body mass (3.8% loss of body weight) than in strength of the leg (increase in strength in proportion to resistance of the weight).29 Although the players who participated in this study lost body mass, that is, they were dehydrated in the first session where the water intake was ad libitum, but without reaching the same percentage of body weight loss in Viitasalo29 (1.3±0.8 vs. 3.8%), alteration was also observed in the height variable of the vertical jump. For its part in the investigation of Armstrong et al.30 no significant differences were observed between the different hydration conditions in vertical jump height, taking into account that the body mass decreased 2.4±0.4% and 4.8±0.4%, respectively, during their intervention in hypohydrates in approximately 2.5% of body mass, and hypohydrates in approximately 5% of body mass.30

All this makes us think that the treatment carried out during these sessions could be enough to cause changes in the replacement of fluids of the players and as a consequence in voluntary dehydration during the competition and therefore in their sports performance. However, we must be aware that these positive results do not imply a change in the habits of these players and that, by not applying the proposed strategies, players may return to their usual hydration patterns. Therefore, we understand that field supervision and fluid supply activities during these sessions should be part of the usual practice during training and competition of a professional team and insist on the need to control as many variables as possible affect athletic performance. We must be rigorous both during training and in competition with regard to the replacement of liquids to avoid the possible negative effects of dehydration.31

Due to the climatic conditions and planning of the sessions, based on the football season, the research had the limitation of occurring on days with different environmental temperature characteristics, although it was possible to compensate efficiently the sweat losses, it would be interesting to know what the behavior would have been like if the weather conditions would have been similar. In addition, there was a small sample, since being a affiliate team players can be promoted to the professional team, if necessary, and participate in different trainings, which was considered within the exclusion parameters. Finally, during the dates planned for the execution of the tests, two players of the staff were injured, therefore, they could not participate in the regular training.

In conclusion, avoiding dehydration through a personalized recommendation guideline and based on the previous measurement of the sweating rhythm improved the jumping height and reduced the error rate of the penalti kick with visual signal, when compared with a rehydration ad libitum. The accuracy of the penalty kick was reduced by fatigue induced by the state of dehydration, especially with the decision-making variable. These data could indicate that a moderate dehydration could affect the muscular strength of the leg and reduce the motor precision during a simulated soccer penalty kick. In addition, the data suggest that using rehydration according to the thirst stimulus may not be the best strategy to avoid reductions in the physical and technical performance of soccer players.

The research was solely funded by the corresponding authors’ institution. The results are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.