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Oddsson, HR, Friðgeirsdóttir, KÝ, Hafliðadóttir, L, Einarsson, IÞ, Kristjánsdóttir, H, and Saavedra, JM. Differences in anthropometric parameters, physical fitness, and kicking speed in young football players according to performance level, playing position, and relative age effect: a population-based study. J Strength Cond ResXX(X): 000–000, 2024—The objectives of this study were to determine (a) the differences, both in male and female players, in anthropometric parameters, physical fitness, and kicking speed based on the players' level and position on the field; (b) whether there is a relative age effect based on the players' level, and (c) whether there is a relationship between the relative age effect and the anthropometric parameters, physical fitness, and kicking speed parameters. Eight hundred twenty-six football players (15.76 ± 2.60 year old, 75.7% male) participated in the study. Players were grouped into elite and nonelite, as well as based on their field position and birth quartile. Nonparametric tests were used to study differences by level and by position or trimesters. Chi-square goodness of fit analysis was performed to know the association between elite and nonelite players regarding the relative age effect. Physical fitness parameters and kicking speed were higher in elite players than in nonelite players (5.834 ≥χ2≤ 48.079; p< 0.001). There were differences in anthropometric and physical fitness parameters and kicking speed depending on the player's position: goalkeeper vs. outfield players (7.648 ≥χ2≤ 26.078; p< 0.001) except in jump tests. There was a relative age effect in elite male players (χ2= 26.313; p< 0.001) but not in female players, and it was related to anthropometric and physical fitness parameters. This population-based study offers insights into the variables that differentiate elite from nonelite players at young ages, with differences in the relative age effect between male and female players, which can be used by coaches to identify key performance variables.