## 1 Introduction  
The purpose of this study was to compare the change of Body Composition (BC) (height, weight, body fat percentage and muscle mass) as a function of the trimester and category in a sample of young soccer players. Data collection was performed in five consecutive seasons (2016-2021). The sample consisted of 741 young male football players of different categories (Under 14 year old (U14), U15, U16, U17 and U18) belonging to a high-performance football academy. Considering the trimestral change of all the raw anthropometrics variables a set of new variables called the trimestral change in percentage (TC) of each raw variable was computed. Two-way repeated measures ANOVA (including the raw anthropometric variables as dependent and trimester and the age-category as independent) revealed differences for the anthropometric variables (\(p\) value \(<\) 0.001 in all cases), concluding that the effect of trimester reaches conventional levels of statistical significance. The trimester by age in contrast was significant (\(p\)\(<\) 0.05) in all raw variables except for the height. Considering the TC variables, the variable height-TC showed an increase (\(p\) value \(<\) 0.05) while the variable muscle mass-TC was near the significative value (\(p\)\(=\) 0.09). In this case the interaction trimester by age category was not significative (\(p\)\(>\) 0.05 in all cases). It seems that height suffers more changes in the first trimester but the weight, body fat percentage and muscle mass changes more in the second and third trimester. It is important to modulate the training load according to the trimester-specific response, although these improvements may vary according to factors such as genetics, diet, sleep and the specific training. young soccer players; changes; anthropometric measurements  
## 1 Introduction  
Football and all the parameters that affect performance (conditional, technical, tactical, age, gender, etc. ), have evolved over the years towards a more specific and individual work from the area of professional performance and areas of training and talent detection [1]. However, few studies focus on the body composition (BC) variables for the direct assessment of performance in football and the adaptations produced by the different training trimesters during the season [2]. Although it is a relatively recent assessment method, there are studies that attempt to define the individual characteristics of players in terms of BC measured with electrical impedance [3,4]. BC with impedance assessment is commonly used to track the improvement of elite football players throughout the regular season [5]. However, the BC and anthropometric characteristics of young players are little considered in the selection of young football players, as the focus is often placed on the skills that each individual has in their respective specialty [6] or much more time is devoted to increase the physical aptitudes of athletes without taking into account the evaluation of their BC and nutritional status [7], even though we know that there are currently differences in terms of physiological demands according to the position of the player and with respect to their weight, height and body mass index (BMI) [8]. If we refer to the study of BC in football and its relationship with health and injuries, BMI is commonly referred to instead of Body Fat Percentage (BFP), however, it has been determined that abdominal fat and circumference, as well as overweight indicated in BFP are better discriminators of injury risk and health status than BMI [9,10]. BMI does not distinguish between fat-free mass, where we include muscle mass (MM) or bone, and body fat, nor does it distinguish between fat distribution, knowing that abdominal fat, especially intra-abdominal fat, and fat in the gluteal-femoral region may even have a greater impact on health [11,12].