Selection of reference data significantly influences biological maturity timing classification in national youth soccer players

This study investigated biological maturity timing (BMT) in 116 Irish U13 and 958 Swedish U15 male national soccer players using various growth reference datasets. BMT was expressed as a Z-score comparing each player's percentage of predicted adult height (%PAH) to age-specific means and standard deviations reported in the: Berkeley Growth Longitudinal (US), UK 1990 growth reference (UK), and Swedish 2000s growth reference (SWE) studies. Multinomial logistic regression was used to compare BMT classification (Early, On-time, Late) between datasets. One-sampled means t-tests were used to examine BMT-related maturity biases per cohort and reference dataset. For both Swedish U15 ( p < 0.001) and Irish U13 ( p = 0.003) players, BMT classification was significantly influenced by the selection of the growth reference dataset. Only 61% of players had the same BMT category across all reference datasets. Significant maturity biases existed in favour of early maturing Swedish players (mean = 96.5%PAH) in all datasets, although moderate using SWE ( d = 0.7), large using UK ( d = 1.02), and very large using US ( d = 1.36) (all p < 0.001). Small significant maturity biases existed in favour of early maturing Irish players (mean = 87.1%PAH) using UK ( p < 0.001, d = 0.48) and US ( p < 0.001, d = 0.33), but not SWE. We conclude that maturity Z-scores must be interpreted with significant caution, given the large variations in BMT classification between reference datasets.