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

Manuel Becker,Christoph Zinner,Billy Sperlich,Silvia Achtzehn

doi:10.3389/fphys.2020.00838

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-wkmid) and at the end (3-wkend) 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-wkmid and 3-wkend each Monday, Wednesday, and Friday. In 3-wkmid the CK median was 14% higher (241 vs. 212 U/L) compared to 3-wkend (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-wkmid did not differ to the values of 3-wkend. 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.

Highlights

Soccer players under the age of 19 (U-19) are exposed to frequent training sessions and matches, which are scheduled in one- or two-match weeks
Results from Fri6 were excluded in Tables 2A,B because all players had two successive days off on the previous days and external load was different before Fri6 compared to the other testing days
The aim of the present study was to compare the results of two selected time periods within a soccer season, to investigate the inter-day and inter-week variability of selected biomarkers (CK, urea, and C-reactive protein (CRP)) and to investigate the inter- and intra-individual kinetics of each variable

Summary

Introduction

Soccer players under the age of 19 (U-19) are exposed to frequent training sessions and matches, which are scheduled in one- or two-match weeks. It is well known that high match and frequent training session exposure in youth soccer players is associated with increased injury risks (Brink et al, 2010; Pfirrmann et al, 2016). Monitoring internal and adjusting external load may ensure optimal training and match performances as well as prevent injuries over the course of an entire season. Monitoring in Elite U-19 Soccer Players expert opinion warrants objective and reliable methods (i) to adjust external load for optimal adaptation and prevention of overuse and (ii) to quantify each player’s fatigue state (Nedelec et al, 2012). Dense biomarker monitoring is warranted in order to interpret time-courses of concentrations since most biomarkers are detectable up to several days (Ascensao et al, 2008)

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