Intra-Individual and Seasonal Variation of Selected Biomarkers for Internal Load Monitoring in U-19 Soccer Players

Manuel Becker; Billy Sperlich; Christoph Zinner; Silvia Achtzehn

doi:10.3389/fphys.2020.00838

Intra-Individual and Seasonal Variation of Selected Biomarkers for Internal Load Monitoring in U-19 Soccer Players

Front Physiol. 2020 Aug 4:11:838. doi: 10.3389/fphys.2020.00838. eCollection 2020.

Authors

Manuel Becker^{1

2}, Billy Sperlich³, Christoph Zinner⁴, Silvia Achtzehn^{5

6}

Affiliations

¹ Institute of Exercise Training and Sport Informatics, German Sport University Cologne, Cologne, Germany.
² Decode Dynamics Sports Performance Institute, Waldbronn, Germany.
³ Integrative and Experimental Exercise Science and Training, Institute of Sport Science, University of Würzburg, Würzburg, Germany.
⁴ Department of Sport, University of Applied Sciences for Police and Administration of Hesse, Wiesbaden, Germany.
⁵ Institute of Cardiology and Sports Medicine, German Sport University Cologne, Cologne, Germany.
⁶ The German Research Centre of Elite Sport, German Sport University Cologne, Cologne, Germany.

Abstract

The aim of this study was to investigate inter-day and -week as well as intra- and inter-individual variation of selected biomarkers in high-performance youth soccer players to assist practitioners interpreting player's internal load to counteract underperformance and unwanted health risks. Eleven male youth soccer players were tested multiple times during two 3-week periods at midpoint (3-wk_mid) and at the end (3-wk_end) of the first half of a German under-19 1. Bundesliga season. The levels of creatine kinase (CK), urea, and C-reactive protein (CRP) were measured during 3-wk_mid and 3-wk_end each Monday, Wednesday, and Friday. In 3-wk_mid the CK median was 14% higher (241 vs. 212 U/L) compared to 3-wk_end (P = 0.26, ES = 0.16). Overall, the medians of CK, urea (P = 0.59, ES = 0.08), and CRP (P = 0.56, ES = 0.10) during 3-wk_mid did not differ to the values of 3-wk_end. Daily coefficient of variations (CVs) ranged from 22 to 71% (CK), 17 to 37% (urea), and 9 to 164% (CRP). Individual medians ranged from 101 to 350 U/L (CK), 23 to 50 mg/dL (urea), and 0.6 to 1.1 mg/L (CRP). High intra-individual variability was demonstrated by large intra-individual CVs (medians: CK 50%, urea 18%, and CRP 45%). Our data show (i) large inter-day and inter-week variability of all biomarkers, depending on the external load and (ii) considerable inter- and intra-individual parameter variations. Creatine kinase concentrations could sensitively reflect soccer-specific loads during the season.

Keywords: biomarker variability; creatine kinase; internal load; monitoring; point of care testing; soccer; youth soccer.