Author: Nils Haller et al
Journal: Nature (2026)

AI generated summary

This study examined how external training load relates to a wide range of internal load measures in elite youth soccer. Twenty-five male players (≈16–17 years) were monitored over three months during the competitive season. External load (e.g., total distance, high-speed running, high metabolic power distance) was tracked via a local positioning system and compared with subjective questionnaires (RPE, drive/energy, sleep quality), blood biomarkers (e.g., CK, LDH, CRP, transferrin), and neuromuscular performance (countermovement jump, CMJ). Training load was analyzed using three common approaches: single-day load (1DL), exponentially weighted 7-day load (7DL), and the acute:chronic workload ratio (ACWR).

Across all calculation methods, subjective measures—especially RPE—showed the strongest and most consistent associations with external load. RPE correlated moderately with distance, high metabolic power distance, and high-speed running, while perceived drive tended to decrease as load increased. In contrast, biomarkers and CMJ variables displayed weaker, less consistent relationships. Among blood markers, lactate dehydrogenase (LDH) emerged as the most sensitive indicator, showing positive associations with load across 1DL, 7DL, and ACWR. CK showed small positive associations mainly with ACWR, while CRP and transferrin tended to relate negatively to certain load metrics. CMJ eccentric force measures were negatively associated with ACWR, suggesting sensitivity to rapid changes in load rather than absolute volume.

Correlations among tools collected at the same time point revealed strong relationships within questionnaires, modest relationships within related biomarkers (e.g., LDH–CK), and limited overlap between subjective and objective measures. Overall, the findings reinforce that well-designed questionnaires are practical and robust tools for day-to-day load monitoring in elite youth soccer, while biomarkers and neuromuscular tests may add value when used selectively and interpreted individually rather than as routine standalone indicators