The international tournaments of national teams represent an important change in the players’ regular workload. After a long season, the training prior to the competition substitutes the vacation period for players to attain optimum fitness. As shown in a study published in 2019 by the medical department in charge of the Australia National Football team,¹ in this training period, the workload is significantly increased, mainly due to a greater number of training sessions. This, plus the workload from the season, make increase the physical demands significantly.

That’s why monitoring fatigue is relevant both in the previous weeks and during the tournament. One of the main procedures to control fatigue is the use of blood markers, such as Creatine Kinase or urea. Even though these parameters are usually used to assess muscle damage or metabolic status, there is high variability among subjects that hinders their accurate interpretation. Moreover, these markers may vary throughout a regular season depending on the type of work done and the physical demands required. Thus, in competitions such as a European Championship or a World Cup, altered values can be frequently found in a regular week of matches that could be considered a warning sign. Therefore, the context is relevant to interpret these markers accurately.

Considering the scarce reference data in this kind of competitions, recently it has been published a research which was carried out to retrospectively analyse fatigue markers in the German National Football team during World Cups and European Championships from 2006 to 2016.² During 3 World Cups (2006, 2010 and 2014) and 3 European Championships (2008, 2012 and 2016) the CK and urea of 68 players were registered (1,019 CK data and 943 urea data).

How Did the Values Vary from 2006 to 2016?

The results showed that the average values of CK and urea were 343 U/l and 39.5 mg/dl respectively, but the variability among subjects was too high, around 45% for both variables. This means that these differences in CK and urea levels emphasise the need to use individualised reference ranges to assess the players’ recovery needs with optimal accuracy.

Furthermore, if we focus on the temporary evolution of these two variables, both the CK and urea showed a significant decrease during the analysed period. According to the authors of this study, “this decrease might be associated with better training and management of the player’s load, which leads to greater tolerance and less fatigue during these tournaments”.

What Relationship is there between External Load and Fatigue Markers?

Changes in the game style and training may produce alterations in the players’ fatigue levels, therefore, the markers should be analysed in relation to other load parameters (external or internal) with the aim to contextualise the data obtained as much as possible. In this case, researchers found a significant association between external load markers, such as the total distance and the total distance covered at high speed, and the CK levels obtained two days after the game. It was also observed, a “disproportionate” response of CK levels when the athletes played matches which lasted more than 90 minutes, possibly due to “muscle microtrauma in a state of fatigue.” However, there was no relation between urea levels and parameters of external load.

Conclusions

Players’ monitoring requires the integration of different markers that help establish a map as broad and complete as possible of their physical and psychological condition. The assessment of fatigue through blood parameters, especially CK, may help analyse the player’s muscle demands. Also, due to the high variability of these markers among players and the importance of the moment in which they are measured, it is necessary that the reference values are individualized and updated regularly.

Adrián Castillo

References:

1. Noor D, McCall A, Jones M, Duncan C, Ehrmann F, Meyer T, et al. Transitioning from club to national teams: Training and match load profiles of international footballers. J Sci Med Sport. 2019;22(8):948–54.
2. Hecksteden A, Meyer T. Blood-borne fatigue markers during major international football tournaments – a retrospective analysis of data from the FIFA World Championships and UEFA European Championships 2006 – 2016. Sci Med Footb. 2020 Apr 2;4(2):135–41.