Author(s): ROKSVAAG, K., ALTMANN, S., HÄRTEL, S., WOLL, A., CUMMING, S., RUF, L., Institution: TSG RESEARCHLAB & KARLSRUHE INSTITUTE OF TECHNOLOGY, Country: GERMANY, Abstract-ID: 2119

INTRODUCTION:
Substantial inter-individual differences in growth and maturity have been observed between players of the same chronological age in male academy soccer. However, such evidence in female youth soccer players is limited. Thus, we aimed to describe the distribution of biological maturity across a sample of ten soccer clubs and playing positions, including both female professional academy and grassroots level players from Germany and Austria.
METHODS:
We estimated the skeletal age (SA) of 437 players (academy=337, grassroots level=100, chronological age (CA) range: 8.4–17.4 years) from the U12–U17 age groups of ten fe-male soccer clubs with a portable ultrasound device (BAUSportTM, SonicBone). The main outcome variable was relative maturity, an indication of biological maturity timing, calculated as SA–CA for each player. The playing position of each player was self-reported. The study was exploratory and SA-CA estimates of each age group or position were deemed equivalent based on the location the 95% confidence intervals in relation to the smallest effect size of interest (SESOI), which was defined as ± 0.5 years.
RESULTS:
For the academy sample, mean ( standard deviation; 95% confidence intervals) SA–CA was close to zero in the U12 (n=44: 0.130.75 years; -0.10, 0.36) and U17 age groups (n=117: 0.190.90 years; 0.03, 0.35), but positive in the U14 (n=95: 0.591.19 years; 0.35, 0.84) and U15 (n=81: 0.731.20 years; 0.46, 0.99) age groups. Notable inter-individual variability in SA–CA were observed within all age groups (ranges U12: -1.47–2.61, U14: -2.63–4.48, U15: -2.27–3.49, U17: -2.55–2.21). Similar findings were observed in the grassroots sample. Considerable differences in the mean values for SA-CA were observed between positions in both the academy and grassroots sample. For in-stance, across all age groups in the academy sample, central defenders presented the highest mean value for SA–CA (n=74, 0.77  0.99 years, 0.54, 1.00) followed by goal-keepers (n=41, 0.71±1.18 years; 0.34, 1.09), whilst the central defensive midfielders (n=35, -0.02±1.11 years; -0.40, 0.36) and offensive midfielders (n=41, 0.03±1.05 years; -0.31, 0.36) presented the lowest mean value.
CONCLUSION:
Our findings reflect current selection processes in female professional academy and grassroots soccer. Based on our SESOI of ± 0.5 years, relative maturity in the high-level sample was equivalent to zero in the U12 and U17 age group. A trend towards positive relative maturity was observed in the U14 and U15. Notable differences for relative maturity were observed across playing positions in both samples. In the high-level sample, relative maturity for central defenders was above the SESOI favouring early maturing players. In contrast, defensive midfielders were within the SESOI region indicating no substantial selection bias. How these maturation-related differences within age groups and between positions affect physical performance and development requires further investigation.