Abstracts

INTRODUCTION

The main Brazilian Soccer Championship has been played with a variation of one game or two per week for seven months. The interval between games may not be enough for an adequate recovery and that can expose the soccer players to an elevated muscle damage¹1. Ispirlidis I, Fatouros IG, Jamurtas AZ, Nikolaidis MG, Michailidis I, Douroudos I, et al. Time-course of changes in inflammatory and performance responses following a soccer game. Clin J Sport Med. 2008;185:423-31.

2. Dupont G, Nedelec M, McCall A, McCormack D, Berthoin S, Wisløff U. Effect of 2 soccer matches in a week on physical performance and injury rate. Am J Sports Med. 2010;389:1752-8. ^{- 3}3. Nedelec M, McCall A, Carling C, Legall F, Berthoin S, Dupont G. The influence of soccer playing actions on the recovery kinetics after a soccer match. J Strength Cond Res. 2014;286:1517-23.. Moreover, soccer is an intermittent and high intensity sport⁴4. Bangsbo J. The physiology of soccer--with special reference to intense intermittent exercise. Acta Physiol Scand Suppl. 1994;619:1-155. presenting many eccentric muscle actions as jumps and direction changes during a match⁵5. Stølen T, Chamari K, Castagna C, Wisløff U. Physiology of soccer: an update. Sports Med. 2005;356:501-36.. Thus, muscle actions are the main causes of skeletal muscle damage⁶6. Clarkson PM, Hubal MJ. Exercise-induced muscle damage in humans. Am J Phys Med Rehabil. 2002;81Suppl 11:S52-69. which increase the permeability of the plasmatic membrane and the release of cytoplasmatic enzymes into the blood stream⁷7. Brancaccio P, Maffulli N, Buonauro R, Limongelli FM. Serum enzyme monitoring in sports medicine. Clin Sports Med. 2008;271:1-18.. Among these enymes, creatine kinase CK has been described as a good marker of muscle damage and the peak CK concentration has been associated with changes in the peak of maximal isometric strength as well as the changes in the knee-joint range of motion⁸8. Nosaka K, Clarkson PM. Variability in serum creatine kinase response after eccentric exercise of the elbow flexors. Int J Sports Med. 1996;172:120-7..

Although CK concentration during a soccer competition has been investigated⁹9. Zoppi CC, Antunes-Neto J, Catanho FO, Goulart LF, Motta e Moura N, Macedo DV. Alterações em biomarcadores de estresse oxidativo, defesa antioxidante e lesão muscular em jogadores de futebol durante uma temporada competitiva. Rev Paul Educ Fis. 2003;17:119-30. ^{, 10}10. Lazarim FL, Antunes-Neto JM, da Silva FO, Nunes LA, Bassini-Cameron A, Cameron LC, et al. The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship. J Sci Med Sport. 2009;121:85-90. and the results have contributed to enabling the adjustment of the training load, it lowered the unbalance between stress and muscle recovery and the injury risk¹¹11. Smith LL. Cytokine hypothesis of overtraining: a physiological adaptation to excessive stress? Med Sci Sports Exerc. 2000;322:317-31.. These studies are contradictory. While some studies reported no changes in CK concentration⁹9. Zoppi CC, Antunes-Neto J, Catanho FO, Goulart LF, Motta e Moura N, Macedo DV. Alterações em biomarcadores de estresse oxidativo, defesa antioxidante e lesão muscular em jogadores de futebol durante uma temporada competitiva. Rev Paul Educ Fis. 2003;17:119-30. ^{, 12}12. Silva ASR, Santhiago V, Papoti M, Gobatto CA. Psychological, biochemical and physiological responses of brazilian soccer players during a training program. Sci Sports. 2008;23:66-72. others showed CK concentration decreases during a competition¹⁰10. Lazarim FL, Antunes-Neto JM, da Silva FO, Nunes LA, Bassini-Cameron A, Cameron LC, et al. The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship. J Sci Med Sport. 2009;121:85-90.. In addition, the absolute values of CK concentration have been used in some of the previous studies which evaluated CK concentration of soccer players¹1. Ispirlidis I, Fatouros IG, Jamurtas AZ, Nikolaidis MG, Michailidis I, Douroudos I, et al. Time-course of changes in inflammatory and performance responses following a soccer game. Clin J Sport Med. 2008;185:423-31. ^{, 9}9. Zoppi CC, Antunes-Neto J, Catanho FO, Goulart LF, Motta e Moura N, Macedo DV. Alterações em biomarcadores de estresse oxidativo, defesa antioxidante e lesão muscular em jogadores de futebol durante uma temporada competitiva. Rev Paul Educ Fis. 2003;17:119-30. ^{, 10}10. Lazarim FL, Antunes-Neto JM, da Silva FO, Nunes LA, Bassini-Cameron A, Cameron LC, et al. The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship. J Sci Med Sport. 2009;121:85-90. ^{, 13}13. Mougios V. Reference intervals for serum creatine kinase in athletes. Br J Sports Med. 2007;4110:674-8. ^{, 14}14. Hammouda O, Chtourou H, Chaouachi A, Chahed H, Bellimem H, Chamari K, et al. Time-of-day effects on biochemical responses to soccer-specific endurance in elite Tunisian football players. J Sports Sci. 2013;319:963-71..

In sports, the use of fixed reference values for CK concentration has been proposed for monitoring training load, these fixed reference values indicate a threefold for injury risk augmented¹³13. Mougios V. Reference intervals for serum creatine kinase in athletes. Br J Sports Med. 2007;4110:674-8. ^{, 15}15. Totsuka M, Nakaji S, Suzuki K, Sugawara K, Sato K. Break point of serum creatine kinase release after endurance exercise. J Appl Physiol 1985. 2002;934:1280-6.. The reference value of physiological parameters are important for interpretation of data¹⁶16. Silva ASR, Papoti M, Pauli JR, Gobatto CA. Elaboração de tabelas de percentis através de parâmentros antropométricos, de desempenho, bioquímicos, hematológicos, hormonais e psicológicos em futebolistas profissionais. Rev Bras Med Esporte. 2012;183:148-152.. However, the use of fixed reference values for CK concentration as indicators of severe muscle damage is made difficult due to rather high inter and intra individual variability⁶6. Clarkson PM, Hubal MJ. Exercise-induced muscle damage in humans. Am J Phys Med Rehabil. 2002;81Suppl 11:S52-69. ^{, 7}7. Brancaccio P, Maffulli N, Buonauro R, Limongelli FM. Serum enzyme monitoring in sports medicine. Clin Sports Med. 2008;271:1-18. ^{, 15}15. Totsuka M, Nakaji S, Suzuki K, Sugawara K, Sato K. Break point of serum creatine kinase release after endurance exercise. J Appl Physiol 1985. 2002;934:1280-6.. Another concurring factor might be the training status of the athletes¹⁷17. Brink MS, Nederhof E, Visscher C, Schmikli SL, Lemmink KA. Monitoring load, recovery, and performance in young elite soccer players. J Strength Cond Res. 2010;243:597-603.. Therefore, the use of a fixed reference value may underestimate or overestimate the presence of muscle damage. Moreover, recently, a joint consensus statement about monitoring training status of athletes suggested the definition of a range of meaningful differences by determination of individual ranges to the markers utilized¹⁸18. Meeusen R, Duclos M, Foster C, Fry A, Gleeson M, Nieman D, et al. Prevention, diagnosis, and treatment of the overtraining syndrome: joint consensus statement of the European College of Sport Science and the American College of Sports Medicine. Med Sci Sports Exerc. 2013;451:186-205..

We have not found studies describing the CK concentration profile of elite soccer players during a competitive season based on individual values. Therefore, the main purposes of the present investigation were a to determine the individual´s profile of blood CK concentration of elite soccer players and b to analyze the CK concentration in different periods during a Professional Brazilian First Division Championship.

METHODS

seventeen players from a first-division Brazilian soccer team volunteered for the study. At the beginning of the season, the age and body mass of the soccer players were registered, 26.6 ± 3.7 years and 77.8 ± 5.6 kg, respectively. At baseline, body mass was measured using standardized procedures, with a calibrated scale Filizola, Brazil, their percent of body fat was estimated by using the Jackson and Pollock¹⁹19. Jackson AS, Pollock ML. Generalized equations for predicting body density of men. Br J Nutr. 1978;403:497-504. equation i.e. sum of chest, axilla, triceps, subscapular, abdomen, suprailium, and thigh skin folds. The VO_2max was estimate by specific soccer test, Yoyo Endurance Test level 2⁴4. Bangsbo J. The physiology of soccer--with special reference to intense intermittent exercise. Acta Physiol Scand Suppl. 1994;619:1-155., with the players using their soccer shoes in the field⁴4. Bangsbo J. The physiology of soccer--with special reference to intense intermittent exercise. Acta Physiol Scand Suppl. 1994;619:1-155.. This test was performed at the beginning of the season.

This study had the approval of the Ethics Committee of the Minas Gerais Federal University, Brazil Protocol 485/10 and all volunteers signed an informed consent form prior to participation in the study.

Procedures

Blood CK concentration was assessed by reflectance photometry at 37° C Reflotron Plus; Roche, Germany, previously calibrated. After the finger asepsis by using alcohol, a 30-µl blood sample was drawn out into a heparinized capillary tube and it was later put on specific reagent strips which were inserted into the instrument. The CK concentration baseline values CK_Bas and the maximal concentration of CK were employed to determine the individual´s profile of blood CK concentration of elite soccer players. The CK_Bas were obtained during the soccer players' presentation after a 30-day rest from any physical activity.

During the competitive season, CK concentration was evaluated between 36 and 46 hours post games CK_Game, a period in which the peak of CK concentration is most likely reached¹1. Ispirlidis I, Fatouros IG, Jamurtas AZ, Nikolaidis MG, Michailidis I, Douroudos I, et al. Time-course of changes in inflammatory and performance responses following a soccer game. Clin J Sport Med. 2008;185:423-31. ^{, 20}20. Fatouros IG, Chatzinikolaou A, Douroudos II, Nikolaidis MG, Kyparos A, Margonis K, et al. Time-course of changes in oxidative stress and antioxidant status responses following a soccer game. J Strength Cond Res. 2010;2412:3278-86.. The maximal concentration of CK_Game found throughout the season, was denominated as CK_Max. In the period encompassing the end of the game and the evaluation of CK_Game, the soccer players simply rested or performed activities considered to be light.

The maximal difference of blood concentration of CK ΔCK_Max was calculated by using the following equation 1:

Therefore, the ΔCK_Max was considered to be 100% and this value was employed to relativize the difference between the CK_Game and CK_Bas. The percentage relation between ΔCK_Game and ΔCK_Max was denominated as %ΔCK_Game Equation 2:

The %ΔCK_Game was thus grouped into quartiles i.e., 1^st, 2^nd, 3^rd, 4^th: ≤25%, >25 and ≤50%, >50 and ≤75%, more than >75%, respectively. This recording procedure of CK was similar to that of Yamin et al. ²¹21. Yamin C, Oliveira J, Meckel Y, Eynon N, Sagiv M, Ayalon M, et al. CK-MM gene polymorphism does not influence the blood CK activity levels after exhaustive eccentric exercise. Int J Sports Med. 2010;313:213-7.. The CK_Game measurements were included only if the soccer players played more than 75 minutes per game²2. Dupont G, Nedelec M, McCall A, McCormack D, Berthoin S, Wisløff U. Effect of 2 soccer matches in a week on physical performance and injury rate. Am J Sports Med. 2010;389:1752-8., did not sustain any muscle strain injury and had not taken any kind of medicine. The occurrence of muscle injury was confirmed by the magnetic resonance imaging MRI Magnetom Vision Plus 1.5 Tesla., Siemens, Germany. Moreover, to be included in the study, the soccer players should have had at least four CK concentration evaluations throughout the season.

The blood CK concentrations of the players were evaluated after each of the 25 official games of the 2010 Brazilian championship from July to December. The CK concentration response during the competition was shown in accordance with the division of the competitive calendar in three periods i.e., initial, intermediate and final. That division was done in accordance with the number of evaluated games, thus each period would have a similar number of evaluations. The initial period was arranged by grouping the first nine analyses of the games which took place between July and August. The intermediate period corresponded to the month of September when the 10^th to the 17th games were evaluated. At the final period, eight games that took place between October and November were analyzed. For this analysis, a higher CK concentration was used for each soccer player from each period. Beyond the inclusion criterion above, for this analysis, there was an inclusion criterion that the soccer players should have at least one CK concentration evaluation in each period and not have remained more than 20 consecutive days without any training at all, which is considered to cause detraining in soccer players²²22. Bangsbo J. Mizuno M. Morphological and metabolic alterations in soccer players with detraining and retraining and their relation to performance. In: Reilly T, Lees A, Davids K, Murphy WJ, editors. Science and Football. Proceeding of the first world congress. Liverpool, England; 1987. p. 11-24..

Monitoring training load

In the training sessions, the workload was recorded by counting the minutes of effective training for each soccer player, to determine the training volume i.e. minutes trained in each period and by the training intensity defined as the heart-rate percentage i.e. %HR_Max Polar Team System^(r); Polar, Finland. The highest heart-rate value reached by the soccer players during training sessions or friendly games was used as their maximal heart rate HR_Max ²³23. Antonacci L, Mortimer LF, Rodrigues VM, Coelho DB, Soares DD, Silami-Garcia E. Competition, estimated, and test maximum heart rate. J Sports Med Phys Fitness. 2007;474:418-21.. The training volume was recorded during games and training sessions. However, the intensity training was recorded only in the training sessions, since according to soccer rules, it is not allowed to use the heart rate monitors in official games. The environmental conditions during the season was recorded according to data supplied by the National Institute of Metereology.

Statistical analysis

The normality of data was analyzed using the Kolmogorov-Smirnov test. The data that have not shown normality were presented as median and amplitude between the minimal and maximal values and the ones that presented normality were expressed as mean and standard deviation. For the analysis of the individual´s profile of blood CK concentration of the %ΔCK_Game frequency in each quartile during the competition, the chi-square X²2. Dupont G, Nedelec M, McCall A, McCormack D, Berthoin S, Wisløff U. Effect of 2 soccer matches in a week on physical performance and injury rate. Am J Sports Med. 2010;389:1752-8. was employed, the chi-square test had as a null hypothesis that every quartile was equal. The comparison among the three periods of the competitive calendar was used through ANOVA one way with repeated measurements followed by the Student Newman Keuls test, in which the highest measurement of %ΔCK_Game for each soccer player in each period was used. To verify whether CK_Game was different from CK_Bas, the paired t-test was used, in which the lowest value of CK_Game of the season was considered. The Sperman correlation coefficient was performed to verify the relationship between changes in training volume, training intensity and CK during the season. The adopted significance level was p<0.05. The data were analyzed using the sigma Stat 3.5 pack. The descriptive analysis of data was expressed in terms of mean, standard deviation, median and frequency distribution.

RESULTS

The percent of body fat %fat and VO_2max of the soccer players were 9.5 ± 1.4 % and 55.5 ± 3.6 mL.kg^{- 1}1. Ispirlidis I, Fatouros IG, Jamurtas AZ, Nikolaidis MG, Michailidis I, Douroudos I, et al. Time-course of changes in inflammatory and performance responses following a soccer game. Clin J Sport Med. 2008;185:423-31..min^{- 1}1. Ispirlidis I, Fatouros IG, Jamurtas AZ, Nikolaidis MG, Michailidis I, Douroudos I, et al. Time-course of changes in inflammatory and performance responses following a soccer game. Clin J Sport Med. 2008;185:423-31., respectively.

For the individual´s profile of CK concentration of elite soccer players, the frequency higher than the expected of %ΔCK_Game was observed in the 2^ndquartile while the frequency in the 1^st and 4^th was lower than the expected one. However, no difference was found between the observed frequency and the expected one in the 3^rd quartile table 1.

The comparison of CK concentration between the periods was performed, made with the data of 12 soccer players. Two soccer players were excluded because of injury and for having remained inactive for more than 20 consecutive days. In addition, three others were excluded for not having at least one CK concentration analysis in each period. In the three analyzed periods during the competition, it was observed that the highest %ΔCK_Game of the intermediate and final periods was lower than the highest %ΔCK_Game of the initial period figure 1. Moreover, it was observed that all soccer players reached the CK_Max between the 1^st - 13^th games value of median in the 5^th game. During the competition, there were four soccer players who sustained muscle injury. Among them, two soccer players had the post-injury CK_Game measured as 577 and above 2000 U/L value above the detectable by the instrument, well above their respective CK_Max of 558 e 573 U/L. Using these measures between periods CK concentration, the standard error of measurement according to Weir²⁴ 24. Weir JP. Quantifying test-retest reliability using the intraclass correlation coefficient and the SEM. J Strength Cond Res. 2005;191:231-40.was 31.1%.

CK_Game was higher than CK_Bas p<0.001 and the CK_Gameshowed a non-normal distribution, with values ranging from 141 to 1830 U/L figure 2. The median of CK_Bas, CK_Max, ΔCK_Max CK_Game, and Δ%CK_Game was 120 U/L range 26.3-475 U/L, 626 U/L range 350-1830 U/L, 478 U/L range 251.8-1355 U/L, 376.5 U/L range 141-1830 U/L and 47.30% range 6.6-100.0% respectively. During the competition, the soccer players had their CK concentration measured 10.8 times on the average.

Moreover, the monitoring of training volume, training intensity and their correlations with %ΔCK_Game are in the table 2. During the all season, their mean playing official soccer match was 1505.9 ± 571.4 min. The mean values for temperature and air relative humidity during the study were 20.9 ± 4.6ºC e 69.0 ± 0.2%, respectively.

DISCUSSION

To the best of our knowledge, this is the first report to determine the individual´s profile of blood CK concentration of elite soccer players therewith, the main finding of this study was the identification of the individual's profile during competition in the 2^nd quartile. Thus, the frequency of %ΔCK_Game at the 2^nd quartile was higher than expected. On the other hand, the observed frequency at the 1^st and 4^th quartiles was lower than expected. Thus, based on the CK concentration response, the soccer players presented muscle damage between 25 and 50 % of the percentage relation between ΔCK_Game and ΔCK_Max along the competition, suggesting that it did not represent an elevated magnitude of muscle damage. However, they did not have a low magnitude of muscle damage and according to the frequencies during the 1^st quartile, were lower than expected. Moreover, there was a decrease of the highest frequency of %ΔCK_Game of each soccer player, in the intermediate and final periods in comparison with the initial period. Considering that in the present study, there was not a monitoring only of intensity of games, the chronic effect of training and games in CK concentration response was also observed, and thus the CK concentration observed decrease suggests that the soccer players had muscle adaptations owing to the stimuli inflicted during the season²⁵25. Maio Alves JM, Rebelo AN, Abrantes C, Sampaio J. Short-term effects of complex and contrast training in soccer players' vertical jump, sprint, and agility abilities. J Strength Cond Res. 2010;244:936-41. Such a fact contributed to the higher observed frequency of %ΔCK_Game at the 2^nd quartile and lower at the 4^th quartile. The profile of the observed %ΔCK_Game corresponded to that of a sample of soccer players of a team that ranked among the top four teams in the championship.

The reduction of CK concentration observed over time in this study may be ascribed to muscle adaptation²⁶26. McHugh MP. Recent advances in the understanding of the repeated bout effect: the protective effect against muscle damage from a single bout of eccentric exercise. Scand J Med Sci Sports. 2003;132:88-97., which has been observed in protocols of eccentric exercise²⁷27. Chen TC, Nosaka K. Effects of number of eccentric muscle actions on first and second bouts of eccentric exercise of the elbow flexors. J Sci Med Sport. 2006;91-2:57-66. and in soccer players during a Brazilian first-division championship¹⁰10. Lazarim FL, Antunes-Neto JM, da Silva FO, Nunes LA, Bassini-Cameron A, Cameron LC, et al. The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship. J Sci Med Sport. 2009;121:85-90.. One of the mechanisms responsible for muscle adaptation can be derived from the activation of myogenic satellite cells that act in the repair of damaged muscle fibers²⁸28. Tidus PM. Skeletal muscle damage and repair. United Kingdom: Human Kinetics; 2008..

According to CK_Max observed in the initial period, the first games of each soccer player may represent more muscle damage than the other games. Therefore, it may contribute for the CK_max that was found in the soccer players in the first games.

Different results were reported by Zoppi et al. ⁹9. Zoppi CC, Antunes-Neto J, Catanho FO, Goulart LF, Motta e Moura N, Macedo DV. Alterações em biomarcadores de estresse oxidativo, defesa antioxidante e lesão muscular em jogadores de futebol durante uma temporada competitiva. Rev Paul Educ Fis. 2003;17:119-30. who did not see any decreases of CK concentration for five months of the soccer championship, but they performed their study during a regional competition with a different competitive calendar and the players belonged to a different competitive level than in the present study. Silva et al. ¹²12. Silva ASR, Santhiago V, Papoti M, Gobatto CA. Psychological, biochemical and physiological responses of brazilian soccer players during a training program. Sci Sports. 2008;23:66-72., who evaluated the CK concentration response during a three-month training period, did not report any differences during that period. However, they performed only three evaluations, one being at the beginning, another 6 weeks later and the last on the 12^th week, each evaluation being performed 12 hours after the last activity. The studies that analyzed the CK concentration during a soccer season evaluated it once a month⁹9. Zoppi CC, Antunes-Neto J, Catanho FO, Goulart LF, Motta e Moura N, Macedo DV. Alterações em biomarcadores de estresse oxidativo, defesa antioxidante e lesão muscular em jogadores de futebol durante uma temporada competitiva. Rev Paul Educ Fis. 2003;17:119-30. ^{, 10}10. Lazarim FL, Antunes-Neto JM, da Silva FO, Nunes LA, Bassini-Cameron A, Cameron LC, et al. The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship. J Sci Med Sport. 2009;121:85-90. or once every six weeks¹²12. Silva ASR, Santhiago V, Papoti M, Gobatto CA. Psychological, biochemical and physiological responses of brazilian soccer players during a training program. Sci Sports. 2008;23:66-72. and they did not take into consideration whether the last physical activity before each evaluation had been a game or a training session⁹9. Zoppi CC, Antunes-Neto J, Catanho FO, Goulart LF, Motta e Moura N, Macedo DV. Alterações em biomarcadores de estresse oxidativo, defesa antioxidante e lesão muscular em jogadores de futebol durante uma temporada competitiva. Rev Paul Educ Fis. 2003;17:119-30. ^{, 10}10. Lazarim FL, Antunes-Neto JM, da Silva FO, Nunes LA, Bassini-Cameron A, Cameron LC, et al. The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship. J Sci Med Sport. 2009;121:85-90. ^{, 12}12. Silva ASR, Santhiago V, Papoti M, Gobatto CA. Psychological, biochemical and physiological responses of brazilian soccer players during a training program. Sci Sports. 2008;23:66-72..

Therefore, the present study was the first, to our knowledge, that evaluated the CK concentration after official games and which had an elevated number of measurements on the same individual. The protocol we used, measuring CK concentration after every game appears to be more adequate to detect alterations in that marker than the protocol used in previous studies.

Throughout the competition the CK concentration of the soccer players remained above the resting and reference values accept for the sedentary population. This corroborates the results of other studies with soccer players during a competition⁹9. Zoppi CC, Antunes-Neto J, Catanho FO, Goulart LF, Motta e Moura N, Macedo DV. Alterações em biomarcadores de estresse oxidativo, defesa antioxidante e lesão muscular em jogadores de futebol durante uma temporada competitiva. Rev Paul Educ Fis. 2003;17:119-30. ^{, 10}10. Lazarim FL, Antunes-Neto JM, da Silva FO, Nunes LA, Bassini-Cameron A, Cameron LC, et al. The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship. J Sci Med Sport. 2009;121:85-90. and studies that analyzed the increase of CK concentration 48h after a single game¹1. Ispirlidis I, Fatouros IG, Jamurtas AZ, Nikolaidis MG, Michailidis I, Douroudos I, et al. Time-course of changes in inflammatory and performance responses following a soccer game. Clin J Sport Med. 2008;185:423-31. ^{, 20}20. Fatouros IG, Chatzinikolaou A, Douroudos II, Nikolaidis MG, Kyparos A, Margonis K, et al. Time-course of changes in oxidative stress and antioxidant status responses following a soccer game. J Strength Cond Res. 2010;2412:3278-86. ^{, 29}29. Ascensão A, Rebelo A, Oliveira E, Marques F, Pereira L, Magalhães J. Biochemical impact of a soccer match - analysis of oxidative stress and muscle damage markers throughout recovery. Clin Biochem. 2008;4110-11:841-51.. Moreover, the median blood CK_Gameconcentrations i.e., 376.5 U/L which corresponded to a %ΔCK_Game 47.30% of the present study were similar to the mean values of plasmatic concentrations of studies with soccer players in competitions⁹9. Zoppi CC, Antunes-Neto J, Catanho FO, Goulart LF, Motta e Moura N, Macedo DV. Alterações em biomarcadores de estresse oxidativo, defesa antioxidante e lesão muscular em jogadores de futebol durante uma temporada competitiva. Rev Paul Educ Fis. 2003;17:119-30. ^{, 10}10. Lazarim FL, Antunes-Neto JM, da Silva FO, Nunes LA, Bassini-Cameron A, Cameron LC, et al. The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship. J Sci Med Sport. 2009;121:85-90. and slightly lower than studies that evaluated the players after a single soccer match¹1. Ispirlidis I, Fatouros IG, Jamurtas AZ, Nikolaidis MG, Michailidis I, Douroudos I, et al. Time-course of changes in inflammatory and performance responses following a soccer game. Clin J Sport Med. 2008;185:423-31. ^{, 20}20. Fatouros IG, Chatzinikolaou A, Douroudos II, Nikolaidis MG, Kyparos A, Margonis K, et al. Time-course of changes in oxidative stress and antioxidant status responses following a soccer game. J Strength Cond Res. 2010;2412:3278-86. ^{, 30}30. Ascensão A, Leite M, Rebelo AN, Magalhäes S, Magalhäes J. Effects of cold water immersion on the recovery of physical performance and muscle damage following a one-off soccer match. J Sports Sci. 2011;293:217-25.. One limitation of the present study was the one assessment of CK_Bas, however, the CK_Bas about the values of sedentary adult men²¹21. Yamin C, Oliveira J, Meckel Y, Eynon N, Sagiv M, Ayalon M, et al. CK-MM gene polymorphism does not influence the blood CK activity levels after exhaustive eccentric exercise. Int J Sports Med. 2010;313:213-7. and soccer players in rest remained¹1. Ispirlidis I, Fatouros IG, Jamurtas AZ, Nikolaidis MG, Michailidis I, Douroudos I, et al. Time-course of changes in inflammatory and performance responses following a soccer game. Clin J Sport Med. 2008;185:423-31..

At the end of the competition, the soccer players in the present study showed a CK_Max of 626 U/L. The CK concentration of 14 soccer players remained below the threshold value for the increased risk of injury suggested by Mougios¹³13. Mougios V. Reference intervals for serum creatine kinase in athletes. Br J Sports Med. 2007;4110:674-8. 1492 U/L and Lazarim et al. ¹⁰10. Lazarim FL, Antunes-Neto JM, da Silva FO, Nunes LA, Bassini-Cameron A, Cameron LC, et al. The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship. J Sci Med Sport. 2009;121:85-90. 975 U/L. It should be noted that the CK concentration of three soccer players was above 975 U/L and the highest being above 1492 U/L, but no clinical problems were reported. Lazarim et al.¹⁰10. Lazarim FL, Antunes-Neto JM, da Silva FO, Nunes LA, Bassini-Cameron A, Cameron LC, et al. The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship. J Sci Med Sport. 2009;121:85-90., reported that in his study, one soccer player whose CK concentration exceeded the proposed reference value was injured. During the present study, four soccer players underwent muscle injury, with two of them presenting CK_Game above their CK_Max, This may suggest the occurrence of a higher rupture of the muscle tissues, resulting in a larger extravasation of CK into the blood stream³¹31. Martínez-Amat A, Boulaiz H, Prados J, Marchal JA, Padial Puche P, Caba O, et al. Release of alpha-actin into serum after skeletal muscle damage. Br J Sports Med. 2005;3911:830-4..

The observed positive correlation between training volume and %ΔCKGame as well as the absence of correlation with training intensities indicates that training volume, likewise type of exercise²¹21. Yamin C, Oliveira J, Meckel Y, Eynon N, Sagiv M, Ayalon M, et al. CK-MM gene polymorphism does not influence the blood CK activity levels after exhaustive eccentric exercise. Int J Sports Med. 2010;313:213-7. might be, among others, one factor that influences the CK concentration⁷7. Brancaccio P, Maffulli N, Buonauro R, Limongelli FM. Serum enzyme monitoring in sports medicine. Clin Sports Med. 2008;271:1-18.. However, only 12% of common variance exists between training volume and CK concentration. The remaining 88% is originated from others variables. Thus, CK concentration should be interpreted along with other variables such as training status and the individual response to %ΔCK_Game.

Due to the high variability found in CK concentration response to exercise in this as well as in other studies¹⁵15. Totsuka M, Nakaji S, Suzuki K, Sugawara K, Sato K. Break point of serum creatine kinase release after endurance exercise. J Appl Physiol 1985. 2002;934:1280-6. ^{, 21}21. Yamin C, Oliveira J, Meckel Y, Eynon N, Sagiv M, Ayalon M, et al. CK-MM gene polymorphism does not influence the blood CK activity levels after exhaustive eccentric exercise. Int J Sports Med. 2010;313:213-7., the CK_Max of soccer players during a competition may not reach the proposed reference values, even knowing that the soccer players are being subjected to elevated muscle damage. Therefore, the use of a fixed CK threshold value may not be warranted. It is likely that different soccer players have different thresholds and the individualized CK concentration profile should be used to monitor each soccer player during a championship.

CONCLUSIONS

The individualized CK concentration responses of the elite soccer players during a Brazilian championship in the present study indicate that they did not remain at elevated muscle damage. One probable muscle adaptation may occur based on reduced CK concentration.

REFERENCES

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