AUTHOR=Becker Manuel , Sperlich Billy , Zinner Christoph , Achtzehn Silvia 

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

JOURNAL=Frontiers in Physiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2020.00838

DOI=10.3389/fphys.2020.00838

ISSN=1664-042X

ABSTRACT=<p>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<sub>mid</sub>) and at the end (3-wk<sub>end</sub>) 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<sub>mid</sub> and 3-wk<sub>end</sub> each Monday, Wednesday, and Friday. In 3-wk<sub>mid</sub> the CK median was 14% higher (241 vs. 212 U/L) compared to 3-wk<sub>end</sub> (<italic>P</italic> = 0.26, ES = 0.16). Overall, the medians of CK, urea (<italic>P</italic> = 0.59, ES = 0.08), and CRP (<italic>P</italic> = 0.56, ES = 0.10) during 3-wk<sub>mid</sub> did not differ to the values of 3-wk<sub>end</sub>. 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.</p>