Original paper
The upper values of plasma creatine kinase of professional soccer players during the Brazilian National Championship

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Summary

The current schedule of the Brazilian Soccer Championship may not give players enough recovery time between games. This could increase the chances of muscle damage and impaired performance. We hypothesized that plasma creatine kinase (CK) activity could be a reliable indirect marker of muscle overload in soccer players, so we sought to identify the reference values for upper limits of CK activity during a real-life elite competition. This study analyzed changes in plasma CK activity in 128 professional soccer players at different times during the Brazilian Championship. The upper limits of the 97.5th and 90th percentiles determined for CK activity were 1.338 U/L and 975 U/L, respectively, markedly higher than values previously reported in the literature. We also evaluated a team monthly throughout the Championship. The upper limit of the 90th percentile, 975 U/L, was taken as the decision limit. Six players showing plasma CK values higher than this were asked to decrease their training for 1 week. These players presented lower CK values afterwards. Only one player with a CK value higher than the decision limit (1800 U/L 1 day before a game) played on the field and was unfortunately injured during the game. The CK activity in all the other players showed a significant decrease over the course of the Championship, and the values became more homogeneous at the end. The results presented here suggest that plasma CK upper limit values can be used as a practical alternative for early detection of muscle overload in competing soccer players.

Introduction

The games of the National Brazilian Soccer Championship generally occur every Saturday/Sunday and Wednesday/Thursday, which means that training and matches overlap. As a consequence, the players, especially the infield ones, may not have enough time to recover between games. This may result in muscular overload, leading to varying degrees of microtrauma in muscle, connective tissue and/or bones and joints. This stress can progress from the initial benign microtrauma stage to a sub-clinical injury that will impair the athlete's performance.1, 2

In the event of muscle damage, cell membranes rupture and some proteins leak out reaching the bloodstream.3 Some of the enzymes and proteins commonly analyzed after exercise-induced muscle damage include creatine kinase (CK), lactate dehydrogenase, aspartate transaminase and myoglobin.3, 4 Of all of these analytes, plasma CK activity seems to be the best indicator of exercise severity and its effect on tissues.4 However, the idea of using plasma CK analysis as an indirect marker of sub-clinical muscular injury in competing athletes has not been explored in the literature.

Doubts about the application of CK analysis are derived from studies suggesting that this measurement is an unreliable marker for histological muscle lesions.5 In addition, CK values measured in individuals exercising to a similar degree showed high variability and a non-Gaussian distribution.6 However, the majority of these studies involved subjects performing specific exercises for defined short periods.7 While these studies make important contributions to the field of exercise physiology, they do not provide enough information on CK levels in actively competing athletes. Such a study could help advance sports science, so we carried out the present investigation with soccer athletes competing under normal conditions.

We hypothesized that plasma CK activity could be a reliable indirect marker of muscle overload for soccer players, so we began by identifying the reference interval for this parameter during a real-life elite competition. We hypothesized further that plasma CK values below the upper limit of the reference interval could indicate an adaptive muscular response. Some studies have suggested that plasma CK activity can arise in the absence of histological lesions, as a consequence of changes in muscle membrane permeability.8, 9, 10 The same muscle membrane alterations that may increase plasma CK activity also affect the release of growth factors by muscle cells,11 which could be one way in which changes in plasma CK activity reflect muscular adaptation. According this point of view the individual plasma CK activity above the upper reference limit may indicate the transition from adaptive microtrauma to a sub-clinical muscular injury, increasing the potential for damage.

Reference intervals refer to the range of values for a laboratory test observed in a specific population, typically described by upper and lower reference limits and containing the central 95% of results.12 In many cases, such as the commonly measured enzymes, low values are unimportant and only the upper limit of the reference interval is used in medical decision-making. In these cases, the one-sided reference interval can be calculated. The 95% reference region would then consist of a single value, i.e., the 97.5th percentile, representing the upper limit cutoff.13

The purpose of the present study was to analyze the distribution of plasma CK activity to determine the upper limits for professional soccer players who were participating in the 2001 Brazilian National Championship. We also tracked CK activity in one team during this Championship to investigate whether it could be used as a marker for muscle overload.

Section snippets

Materials and methods

The analyzed population included active professional soccer players from five clubs belonging to the Brazilian Soccer First Division. To avoid interference with the experiment, athletes who were taking medications or who were injured and not training were excluded from the study. The athletes (n = 134) were 24 ± 4 years old and weighed 74.7 ± 8.0 kg (means ± S.D.). This study was performed with the approval of the Ethical Committee on Human Research (Proc. 019/2004). All subjects provided written

Determination of the upper reference limit

Fig. 1 shows the histogram of plasma CK activity. We found a non-Gaussian, left-skewed data distribution even after the exclusion of the outliers. The majority of the data were concentrated between 150–1000 U/L, with a high variability (200–1600 U/L).

The upper limit values determined for plasma CK activity are shown in Table 2. Different percentiles 97.5th and 90th were calculated due to higher values inherent in the soccer player population compared to the general population. The plasma CK upper

Discussion

In this study we used the IFCC-recommended methodology to analyze the distribution of plasma CK activity in professional soccer players from five clubs belonging to the Brazilian Soccer First Division. The values varied greatly between individuals, and one reason may be physical differences between the athletes. We used these values to determine an upper limit that might be applied when managing athletes’ performance. Our procedure allowed us to derive an upper limit from soccer players during

Practical implications

  • The availability of portable apparatus for this measurement would allow assessment of muscle stress in real-time during a championship.

  • This may allow for the development of optimal physical conditioning plans that take into account the stress limits for individual players and thereby ensure muscle protection.

  • It is important to point out that the specific reference intervals for the portable kits must be determined before application.

  • In addition, the reference values for athletes in other sports

Acknowledgements

The authors thank the Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq) Brazilian agencies for financial support (Pronex-03/09923-2 and CNPq Proc. 523383-96-7). FL Lazarim is the recipient of a Ph.D. grant from Capes. FOC da Silva is the recipient of a Ph.D. grant from FAPESP (Proc. 04/07203-5). The authors also thank the biologist Ana Maria Marçal Porto for technical support, the physiologist Prof. Helio

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    The 97.5th percentile upper reference limit for CK concentration for professional AF players of 1715 U L−1 was 6.9 times higher than the same ULN for a local population adult male reference range (250 U L−1),22 3.4 times higher than age and gender matched non-athletes (391 U L−1)21 and 3.3 times higher than swimmers (523 U L−1).6 Interestingly, the ULN in the current study were very similar to soccer players (97.5th percentile 1338–1492 U L−1 and 95th percentile 1327 U L−1).6,16,17 The negatively skewed distribution pattern (Fig. 1) was similar to the distribution of CK concentration in the non-athletic population21 and the mixed athlete population.6

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