Asymmetries in lean mass, balance, and stability in 12-year-old female soccer (football) players: a cross-sectional study

Abstract

Background: Physical asymmetries in soccer (football) are inconsistently reported, and data on female youth players remain limited. This study evaluated asymmetries in leg lean mass, dynamic balance, landing kinetics and postural stability among 12-year-old female soccer players. Methods: Eighty-five players from 10 football clubs in Reykjavik, Iceland, participated between April and June 2024. Leg lean mass was assessed via dual-energy x-ray absorptiometry (DXA), dynamic balance using the Y Balance Test (YBT), and single-leg landing test with VALD force plates. Asymmetry between the preferred kicking leg (PKL) and the non-preferred kicking leg (NPKL) was assessed with paired t-tests. Absolute asymmetry magnitudes were compared across coach-rated performance groups using Welch's ANOVA. Results: Participants had a mean age of 11.7 ± 0.3 years, height of 154.4 ± 6.9 cm, and weight of 44.2 ± 8.1 kg. No significant asymmetries were found between leg lean mass (p = 0.197), landing force (p = 0.905) or time to stabilization (p = 0.083). However, significant asymmetry was observed in anterior reach on the YBT (p = 0.008, d = 0.29), favoring the NPKL. No differences were seen in other YBT directions. Discussion: While lean mass and landing performance were symmetrical, the anterior reach asymmetry may reflect early neuromuscular differences. These results suggest that asymmetries in young female players may begin to emerge in select functional domains, even in the absence of structural or kinetic differences. Ongoing monitoring during adolescence may help clarify how training exposure and development influence these patterns.