# On the relationship between external and internal load variables in elite youth soccer players

## TL;DR

This study explores how external and internal training loads relate in elite youth soccer players, finding that subjective measures like RPE are more consistent indicators of training load than biomarkers or neuromuscular tests.

## Contribution

The study provides new insights into the relationship between external and internal load variables in elite youth soccer players using a combination of subjective, biochemical, and neuromuscular measures.

## Key findings

- Subjective measures like RPE showed stronger and more consistent correlations with training load than biochemical or neuromuscular markers.
- Creatine kinase (CK) and lactate dehydrogenase (LDH) positively correlated with training load, while transferrin and C-reactive protein (CRP) showed negative associations.
- Neuromuscular testing, such as CMJ eccentric mean force, was negatively correlated with training load variables.

## Abstract

The study investigated the relationship between external and internal training load measures in 25 male elite youth soccer players (age: 16.6 ± 0.9 years, VO2max: 59 ± 4 ml/min/kg) over 3 months. External load (i.e., total distance, high metabolic power distance, high-speed running) was quantified using a local positioning system and related to subjective (RPE, sleep quality, drive (energy level)), biochemical (creatine kinase (CK), lactate dehydrogenase (LDH), C-reactive protein (CRP), urea, cortisol, transferrin), and neuromuscular (CMJ) markers. Single day workload (1DL), exponential 7-day workload (7DL), and the acute: chronic workload ratio (ACWR) were calculated. 1DL parameters were correlated (Spearman’s rho) with RPE (range r = 0.24 to 0.43, p < 0.01) and 1DL distance was negatively related to drive (r = − 0.28, p < 0.001). LDH correlated positively with training load across all calculation methods (up to r = 0.27, p < 0.01). CK exhibited positive correlations to ACWR training load (r = 0.23 to 0.27, p < 0.05), while transferrin (ACWR) and CRP (1DL) showed negative associations to training load (r = − 0.21 to − 0.28, p < 0.05). CMJ eccentric mean force was negatively correlated with all ACWR training load variables (r = − 0.22 to − 0.25, p < 0.01). In summary, subjective measures showed stronger and more consistent associations with training load than biomarkers or neuromuscular testing. Practitioners may confidently use well-structured questionnaires for load monitoring in elite youth soccer.

The online version contains supplementary material available at 10.1038/s41598-025-31487-z.