We aimed to evaluate the internal consistency and factor structure of the Functional Movement Screen (FMS (TM)) in youth athletes and quantify differences between individual task score at different stages of maturation. FMS (TM) and anthropometric variables were measured in 144 youth athletes (96 female, 48 male). Biological maturation was categorised as before- (<-0.5 years), at- (-0.49-0.50 years) and after- peak height velocity [PHV] (>0.51 years). Internal consistency was poor (Cronbach's alpha; 0.53, +/- 90% confidence limit 0.10; ordinal alpha 0.6, +/- 0.09). Principle component analysis extracted two components, representing 47% of the total variance. Tasks loading highest on component 1 required stability, while those loading highest on component 2 favoured mobility. "Likely" decrements in component 1 tasks were observed before-PHV. In-line lunge (effect size +/- 90% confidence limit; -0.47, +/- 0.49), hurdle step (-0.38, +/- 0.49), and trunk stability push-up (-0.51, +/- 0.45), were lower compared with athletes at-PHV and rotatory stability (-0.44, +/- 0.37) was lower than those after-PHV. Boys' scored "most likely", higher (0.73, +/- 0.28) in trunk stability push-up, and girls "likely" higher in shoulder mobility (0.46, +/- 0.29). In our population, the FMS (TM) is not uni-dimensional, thus total score should be avoided. Clear maturation affects were observed in stability tasks.
