Elite soccer teams regularly compete in varying ambient temperatures. During European club tournaments, such as the Union of European Football Associations (UEFA) Champions League, there can be as much as a 20° C temperature difference between home and away games. It has been demonstrated that hyperthermia plays a crucial role in athletic performance during prolonged endurance exercise (8,30,38). In soccer, both total game distance and distance covered at high intensities are lowered by environmental heat stress (7,24,33). In the studies conducted by Ekblom (7) and Ozgunen et al. (33), games were compared where the temperature difference was only 10 and 2° C, respectively. Moreover, performance was solely evaluated by assessment of the activity pattern by match analysis, which has been shown to be influenced by several components such as standard of play, the opponent, the surface and technical and tactical elements (1,21,35). Performance tests before and after a game have been completed in temperate settings to provide direct evidence of game-induced fatigue (12,14,23), but limited information exists on soccer match play in the heat. Finally, the majority of the aforementioned studies applied experimental games to examine the effect of environmental temperature on match performance. The effect of heat stress during high-level official international games in tournaments such as the UEFA Champions League has not yet been investigated.
Fatigue accumulates during a soccer game, which leads to a deterioration in exercise performance at the end of the game (22,37). Recently, it was demonstrated in an experimental game played in warm environmental conditions that the decline in high-intensity running toward the end of the game appeared to be greater (24) in comparison with comparable games at neutral temperatures (1,21). In addition, repeated jump and sprint performance was lower after the game in comparison with resting conditions. However, no control game was included in the study by Mohr et al. (24) to isolate the effect of hyperthermia on fatigue development.
During exercise in the heat, very high sweat rates are observed, and heat loss by evaporation becomes the central mechanism for heat dissipation (18). In a tennis game in a warm ambient environment, sweat rates >2.5 L·h−1 have been reported (5), and in soccer matches played in severe heat, players can lose >3 L of fluid (26). This would correspond to approximately 4% of the body mass for an average player, which impairs high-intensity exercise performance (17,18). Thus, increased dehydration may deteriorate physical performance in the last stage of a soccer game in warm conditions.
Numerous studies have applied countermovement jump (CMJ) tests to evaluate postgame performance (2,10,14,24,32). However, reported findings of the effect of a soccer game on peak CMJ ability are equivocal. In a study by Mohr et al. (24), peak jump performance was unchanged after a test game played at approximately 30° C, but repeated jump performance significantly deteriorated. Additionally, repeated sprint performance is more compromised by a soccer game than peak sprinting speed (12,23,24). Therefore, single explosive muscle contractions appear to be unaffected by a soccer game, whereas repeated intense exercise performance is markedly lowered. In this study, a repeated CMJ test was applied to obtain rapid measurements after that game and to be able to differentiate between peak and mean CMJ performance.
One study has identified a relationship between Yo-Yo intermittent recovery test performance and high-intensity running in the last 15 minutes of the game played in heat (24), indicating that this test predicts fatigue resistance in the final stage of a soccer game in a hot environment. The players in that study were acclimatized to soccer match play in the heat. Thus, it is currently unknown if physical capacity is associated with fatigue development after a soccer game in heat stress in players normally competing at temperatures around 10° C.
Thus, the purpose of this study was to examine repeated and peak jump performance and fluid loss after UEFA Champions League games played in hot environmental settings compared with temperate conditions in elite male soccer players. An additional aim was to study whether different types of intermittent fitness tests predict the degree of fatigue after a soccer game in the heat.
Experimental Approach to the Problem
To investigate the effect of environmental heat stress on repeated explosive exercise performance and the degree of dehydration after a soccer game, elite players performed a repeated CMJ test in rested pregame condition and after 6 UEFA Champions League (CL) games played at either temperate or high environmental temperatures. Different intermittent fitness tests were additionally conducted before the experiment to evaluate the relationship between physical capacity and the degree of fatigue after a soccer game in the heat.
Nineteen male elite soccer players from a Scandinavian top league took part in the study (age: 26.7 ± 1.0 years, height: 181.7 ± 1.1 cm, weight: 75.8 ± 1.0 kg). The players were representing the league champion teams in 2 different competitive seasons. Thirteen of the players were A level National team players, and 2 represented a U21 National team. All the players had participated in European club team tournaments before the study. The players were professional and semiprofessional players training or playing on average 7 times·wk−1. Written informed consent was obtained from the players before the study, and the study conformed to the code of ethics of the Declaration of Helsinki and adhered to the human subject guidelines of the Department of Exercise and Sport Sciences, Section of Human Physiology, University of Copenhagen, Denmark.
The players representing the local league champions 2005–2006 and 2008–2009, respectively, played in qualification for the UEFA CL 2006 and 2009. In the first and second rounds of the qualification, the 2006 champions met 2 teams from Southern Europe. This was also the case for the 2009 champions in their first qualification round. The 3 home games were played at normal environmental temperatures (CON: 12.2 ± 0.5° C), whereas the away games were played under environmental heat stress (HOT: 30.0 ± 0.3° C). The CON and HOT games were separated by 1 week and were played at approximately the same time of day. All the games were played late in July, which was midseason in their league, and the players had their peak Yo-Yo IR performance of the season in this period.
The players performed a repeated jump test in resting conditions 2 days before the first game (Baseline) and immediately after the home and away game. All the tests were performed at approximately the same time of day, and sleeping patterns and nutritional and hydration procedures were similar during the days before the tests. The 2 games against the same opponent were separated by 7 days. The repeated jump test consisted of 5 maximal jumps on a jump mat separated by 5 seconds (Time It, Eleiko Sport, Halmstad, Sweden). The jumps were initiated from a stationary standing position, but a preparatory countermovement consisting of a 90° knee flexion was allowed. The players were instructed to keep their hands on the waist as previously described by Mohr et al. (24). All the subjects were familiarized with the testing protocol by at least 3 trials. The test has a high reproducibility (coefficient of variation < 2%; unpublished data from our laboratory). The test was carried out on the field or in the dressing room without shoes within a 5-minute period after the game. The test was solely performed on players playing >85 minutes of the game with players tested in the same order after the CON and HOT games. For the 14 players taking part in >1 game, average values represented the test results in the postgame test.
The player's total fluid loss was determined by weighing the players before the warm-up and after the game (JWI-586, Jadever Scale Co. Ltd., Wu-Ku Hsiang, Taiwan) taking into account the fluid intake, which was measured during the game. The fluid intake was assessed by using individual drinking bottles. The sweat rate was calculated based on the total fluid loss correcting for urine loss.
To evaluate possible relationships between physical capacity and the response to soccer match play in the heat, various fitness tests were conducted. The players' physical capacity was assessed with (a) a 6-minute submaximal Yo-Yo intermittent recovery test, level 1 (Yo-Yo IR1 [4,11]) with heart rate (HR) measurements (Polar Electro Oy, Kempele, Finland) where the test result was the HR after the 6 minutes expressed relative to HRmax, (b) a Yo-Yo intermittent recovery test, level 2 (Yo-Yo IR2 [4,6]), and (c) a repeated sprint test (3 × 30-m sprint test with 25 seconds of active recovery as described by Mohr et al. [23,24]) using timing gates (Time It, Eleiko Sport, Halmstad, Sweden). The HRmax was determined by a full Yo-Yo IR1 test (11). The fitness tests were all conducted within 1 week before the first game, and all the players were familiar with the testing procedures.
Values are presented as mean ± SEM. Differences in peak and repeated jump performance between Baseline, post CON, and HOT was determined using a 1-way analysis of variance test. Differences in net loss in body mass and sweat rate between CON and HOT were determined using Student's paired t-test. Relationships between selected variables were evaluated using Pearson's product moment test. Significance level was set as p ≤ 0.05.
The players had a Yo-Yo IR2 test performance of 1,032 ± 42 m (range: 920–1,400 m). The maximal HR was 196 ± 2 b·min−1. The HR was 164 ± 3 b·min−1 after the 6-minute submaximal Yo-Yo IR1 test, corresponding to 83.7 ± 1.1% of HRmax (range: 76.3–88.5%), and a repeated sprint test performance (average sprint time) of 4.38 ± 0.04 seconds (range: 4.15–4.54 seconds). The fatigue index in the repeated sprint test was 4.1 ± 0.3% (range: 1.8–6.0%).
Countermovement Jump Performance
Repeated CMJ performance after HOT was 6.0 ± 0.8% lower (p < 0.05) than Baseline and tended (p = 0.08) to be lower than after CON (Figure 1A). No significant difference was observed in the repeated CMJ ability between Baseline and after CON (Figure 1A). Peak CMJ performance was not different between Baseline and either after CON or HOT (Figure 1B).
Fluid Loss and Sweat Rate
The net loss of body mass was 3.1 ± 0.3% in HOT, which was higher (p < 0.05) than in CON (1.7 ± 0.2%; Figure 2A). The sweat rate in HOT was 2.2 ± 0.1 L·h−1, which was higher (p < 0.05) than in CON (1.3 ± 0.1 L·h−1; Figure 2B).
The relative decrease in repeated CMJ performance from Baseline to after HOT was correlated (r = 0.60; p < 0.05) to the relative net loss of the body mass during the HOT (Figure 3). The net loss in body mass during HOT and CON was correlated (r = 0.53; p < 0.05), and the sweat rate during the HOT and CON games (r = 0.51; p < 0.05). No correlations were found between any of the performance test variables and neither game-induced decrease in jump performance nor the net loss of body mass in HOT and CON.
This study is the first to examine the effect of playing official UEFA Champions League games in hot compared with temperate settings on explosive exercise performance. We demonstrated that repeated CMJ ability decreased significantly from baseline values after a game in the heat. However, after a game in normal ambient temperatures, there was no decline in CMJ ability. Moreover, the heat-induced decrement in repeated jump performance correlated to the net-fluid loss during the game in the heat.
Repeated jump performance markedly deteriorated after games played at approximately 30° C but not after games played at approximately 10° C, indicating a higher degree of physiologically mediated fatigue after elite football games in the heat. Some studies have demonstrated that CMJ jump performance is unaffected immediately after a soccer game in temperate environment (10,14), whereas others have shown a reduction of approximately 3% (2). Moreover, repeated sprint performance is reported to be lowered by 2–3% after a game in temperate conditions (12,14,23). In this study, repeated CMJ ability decreased by 6% after the game in the heat. This decrease is slightly lower than the decrease in repeated jump ability observed after a game played at similar temperatures (∼30° C) in heat-acclimatized players (24). Taken together, these studies suggest the impairment of repeated explosive muscle actions is greater after a game played in the heat than in neutral ambient temperatures. Fatigue patterns in high-level soccer games are well described (22,37), and it has been demonstrated that high-intensity running and sprinting is reduced toward the end of a competitive game (20,21). Recently, a marked drop in high-intensity running was observed in the final 15 minutes of a soccer game played in a hot environment (24). Additionally, the decline in work rate expressed as a game fatigue index was considerably larger than in comparable groups of players competing at a normal environmental temperature (1,21). This observation supports the findings in this study of a higher degree of fatigue at the end of elite competitive games played in hot compared with neutral ambient temperatures.
Fatigue in soccer is a multifacetted phenomenon involving a complex interplay between numerous physiological mechanisms (22,37). In this study, a relationship was found between the game-induced decrease in repeated CMJ performance during the HOT game and the relative net-fluid loss. This is consistent with the findings of an association between a fatigue index in a repeated sprint test and net-fluid loss after a game played at temperatures of approximately 30° C (24). In this study, the net-fluid loss during HOT was 3.1% of the body mass, which is consistent with the findings from previous studies when soccer was played in hot environmental conditions (26,36). Additionally, fluid loss in our present study was higher than that during the control game (1.7%) and values reported in similar studies under temperate conditions (12,23). It is well known that a fluid loss corresponding to 2–7% of the body mass impairs exercise performance during prolonged exercise (3,38). Moreover, moderate fluid loss (2.7%) has been demonstrated to impair 5- and 10-m sprint performance (17) and soccer-specific coordination ability (19). Thus, part of the fatigability in the HOT game may be related to dehydration.
Anaerobic performance has been shown to be negatively affected by dehydration (16). Therefore, the underlying physiological mechanism provoking the deterioration in repeated jump performance after the hyperthermic game may have been caused directly by the dehydrated state. In addition, dehydration also increases the rate of body temperature increase during exercise in the heat (28). Hyperthermia, does not appear to affect force production during brief maximal contractions (27,40), however, during a sustained contraction the capacity to maintain voluntary force production is impaired (29,30). Therefore, these studies support our findings that peak CMJ performance remains unchanged, but hyperthermia-induced dehydration resulted in reduced ability to execute repeated maximal jumps. Elevated muscle temperature (∼39° C) has been shown to promote repeated sprint ability (23), but, in a study by Nielsen et al. (28), dehydration produced an inverse relationship between high-intensity exercise performance and muscle temperature. Moreover, repeated sprint performance after prolonged intermittent work in the heat is attenuated despite a markedly elevated muscle temperature (6). The increase in muscle temperature has been suggested to relate to a hyperthermia-induced reduction in central activation (30,31) or alternatively disturbances in the cerebral neurotransmitter homeostasis (25,29). In this study, neither core nor muscle temperature was measured; however, others have reported markedly higher body temperature values after soccer games in a warm environment (7,15,24) compared with temperate conditions (23). Thus, it appears likely to propose hyperthermia as a potential direct or indirect cause of a greater degree of fatigue in the HOT game.
The sweat rate in HOT was markedly higher compared with CON, slightly higher than previous observations during a soccer game in the heat (15) and similar to a tennis game in hot and humid conditions (5). Turkish soccer players acclimatized to heat stress were demonstrated to have average sweat rates <2 L·h−1 (15), which is of the same magnitude as reported during a training session in the heat (39), however slightly less than in this study. A possible reason for the discrepancy in sweat rate between the present and previous studies may be attributed to the great importance of the games, which may have increased the game intensity and therefore also the sweat rates. Alternatively, there may be differences in training status between the subjects in the studies because trained individuals demonstrate higher sweat rates (38). The players in this study had a training status based on their submaixmal Yo-Yo IR1 and Yo-Yo IR2 test scores, which were comparable with top-class players (4,12). There were large individual variations in sweat rates (range: 1.2–3.4 L·h−1), with 3 players demonstrating values >3 L·h−1. Sweat is invariably hypotonic compared with body fluids; therefore, dehydration because of high sweat loss results in changes in the electrolyte homeostasis (17,18), which may deteriorate optimal muscle function. However, it is unknown to what extent this change in electrolyte homeostasis affects repeated explosive muscle work. It has been suggested that the electrolyte loss, mainly sodium, during exercise is likely to induce muscle cramps (18), but clear evidence is still required.
Hyperthermia has also been shown to cause cardiovascular and metabolic alterations during exercise (9,18). For example, blood lactate levels and muscle glycogen use have been reported to be higher in the heat (34). Partly or fully depleted muscle glycogen stores in individual fibers are considered a likely cause of fatigue in the final stage of a soccer game (12). Reduced muscle glycogen levels may additionally affect the degree of microinjuries in the muscle fiber and SR Ca2+ handling (13). Thus, hyperthermia may reinforce the depletion of muscle glycogen stores, which may have induced a greater fatigue response after the game.
No correlation was found between performance in the different fitness tests and the decrease in repeated jump performance in HOT, which is in agreement with recent data (findings from pilot research from our laboratory), indicating that the degree of fatigue after playing a soccer game in a hot environment cannot be predicted from intermittent-exercise capacity. However, both net sweat loss and sweat rate from the CON game showed a relationship with the HOT game (SRHOT = 1.43 × 0.57 SRCON, r = 0.50), indicating that assessment of sweat rates during games in temperate environments partly estimates fluid loss during soccer match play in the heat.
In conclusion, this study is the first to demonstrate that repeated CMJ test performance declines after official competitive elite soccer games in the heat compared with normal environmental conditions. Finally, the greater degree of fatigue in the heat partly associates with an increased degree of dehydration but cannot be predicted from intermittent fitness testing.
Heat stress and dehydration impair the ability to perform repeated explosive muscle contractions in the final stage of official international games in well-trained elite soccer players. To be able to perform explosive exercise tasks demanding a high rate of force development is essential for game success in multiple sprint sports such as soccer; thus, strategies to counteract hyperthermia-induced fatigue should be developed. This type of heat stress induced fatigue may to a certain degree be provoked by dehydration and concomitant hyperthermia. Therefore, special attention should be given to developing optimal hydration and heat acclimatization strategies before and during soccer games in hot environmental conditions so that performance decrements can be limited. Large individual variations in sweat rates and fluid loss were observed for elite soccer players in a game in the heat, which highlights the importance of determining the athletes that are most likely to become severely dehydrated during competition and training in heat stress. Finally, intermittent fitness test performance such as Yo-Yo intermittent recovery test scores do not appear to be a good predictor of fatigability at the end of a soccer game played in the heat.
The effort of the players taking part in the study and the support of the clubs and technical staff are acknowledged. The technical assistance from Sigfríður Clementsen, Bárður Olsen, and Áki Mørk is greatly appreciated, including grammatical advice from Dr. Sarah Jackman. The study was supported by Team Denmark (Team Denmark), The Danish Football Association (Dansk Boldspil-Union), and The Danish Ministry of Culture (Kulturministeriets Udvalg for Idraetsforskning): Grant no. TKIF2007-027.
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