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Individualized reference ranges for markers of muscle recovery assessment in soccer.

Recently an individualization algorithm has been developed and shown to significantly improve diagnostic accuracy of creatine kinase (CK) and urea in endurance sports and Badminton. In this study, applicability and benefit of this algorithm was evaluated using repeated measures data from 161 professional German soccer players monitored during the 2015-2017 seasons. Venous blood samples were collected after a day off (recovered state) and after a minimum of two strenuous training sessions within 48 h (non-recovered state) and analyzed for CK and urea. Group-based reference ranges were derived from that same dataset to ensure a best possible reference for comparison. A z-Test was conducted to analyse differences in error rates between individualized and group-based classifications. CK values for the individualized approach showed significantly lower error rates in the assessment of muscle recovery compared to both a population-based (p<0.001; z-value: -17.01; test-pass error rate: 21 vs. 67%; test-fail: 19 vs. 64%) and a group-based cut-off (p<0.001; z-value: -15.29; test-pass error rate: 65%; test-fail: 67%). It could be concluded that the assessment of muscle recovery in soccer using individualized interpretations of blood-borne markers may offer higher diagnostic accuracy as compared to a population-based as well as a sample specific group-based approach.Key points: Assessing muscle recovery via CK using individualized ranges seems to offer a higher diagnostic accuracy compared to a sample-specific group-based analysis.Using an individualized algorithm seems to be a promising approach to overcome diagnostic problems arising from large inter- and intraindividual variability in blood parameters as it significantly improved the diagnostic accuracy of CK as a recovery marker.As recovery assessment in elite soccer ultimately aims at the accurate detection of differences in the individual player this algorithm seems to offer coaches and sport scientists a more sensitive approach compared to group-specific evaluations.