Identifying relevant bilateral countermovement jump parameters for performance profiling, neuromuscular fatigue monitoring and asymmetry testing in Academy soccer players

Purpose: This study aimed to determine the reliability and relevance of key countermovement jump (CMJ) parameters in Academy soccer players. Methods: Twenty-nine bilateral CMJ force-time variables were collected using dual force plates from 63 Academy soccer players. Two analyses were performed: i) test–retest reliability was assessed in a convenience sample of 20 players across two sessions separated by seven days, and ii) principal component analysis (PCA) was conducted in 47 players using their most representative seasonal trial to reduce data dimensionality. Results: Most outcome-based variables demonstrated acceptable reliability (intraclass correlation coefficient [ICC] ≥ .81; coefficient of variation [CV] ≤ 13.7%). Nine out of eleven time-based variables showed acceptable reliability (ICC ≥ .67; CV ≤ 12.9%). All asymmetry-based variables showed unacceptable reliability (ICC ≥ .11; CV ≤ 66.4%) while symmetry-based variables generally displayed excellent reliability (ICC ≥ .90; CV ≤ 11.8%). The PCA revealed three principal components within outcome- (force and power production during concentric and eccentric phases) and time-based (temporal ratios, concentric duration, and landing characteristics) variables whereas two principal components were revealed for asymmetry- or symmetry-based variables (related to eccentric, concentric, and landing phases). These explained 83.1%, 76.1%, and 81.9% of the total variance for outcome-, time-and asymmetry or symmetry-based variables, respectively. Conclusions: Practitioners involved with bilateral CMJ testing in academy soccer players can rely on the selected parameters presented in our study. Symmetry parameters could be preferred over asymmetry parameters, due to their superior reliability.