@article{BeckerSperlichZinneretal.2020,
  author    = {Becker, Manuel and Sperlich, Billy and Zinner, Christoph and Achtzehn, Silvia},
  title     = {Intra-Individual and Seasonal Variation of Selected Biomarkers for Internal Load Monitoring in U-19 Soccer Players},
  series = {Frontiers in Physiology},
  volume    = {11},
  journal   = {Frontiers in Physiology},
  number    = {838},
  issn      = {1664-042X},
  doi       = {10.3389/fphys.2020.00838},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-209616},
  year      = {2020},
  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.},
  language  = {en}
}