## 4 Discussion  
The purpose of this study was two-fold, to compare the trimestral change percentages for each of the variables analyzed (height, weight, body fat percentage, and muscle mass) as a function of the trimester in a sample of adolescent soccer players and to compare the trimestral change percentages as a function of the variables analyzed. Several studies have reported seasonal variations in the physical fitness and anthropometrical variables of young soccer players [29; 30]. This study revealed a significant increase in all the anthropometrical indicators analyzed from the start to the end of the season. However, it must be recognized that maturity state is a variable that can influence such changes [29]. According to the results found in the present study, it seems that from 13 to 15 years of age, there are greater changes per trimester in the height-TC of the players (although there were no significant differences) (Figure 2). As expected from a previous study of European youth players [31], the highest increases in height and weight occurred when players turned 14 years of age. When analyzing Muscle Mass-TC, it was noticed an interesting nuance, as it turns out that the peak gain in muscle mass occurs a year after the peak gain in body weight [30]. This is possible because fat mass decreases due to rapid growth and testosterone secretion increases while muscle mass increases. Testosterone is known to reduce triglyceride synthesis [32]. Muscle mass as a percentage of body weight improved from 3.09% at age 13 to 3.36% at age 15, which is consistent with results for the general population [31; 32; 33]. According to [29], several physical fitness variables such as limb movement speed, core strength, explosive power, running speed, agility, cardiorespiratory endurance, and anaerobic capacity all show greatest development at peak height, with weight and height gain occurring at the age of 13.8 years. Therefore, it is very relevant to prepare physical exercise programs aimed at improving these physical capacities in this age range. Changes in height, weight, muscle mass between different years have been extensively studied. For example, it is estimated that the peak of height growth in child football players is reached at 13.8 + 0.8 years and peak weight velocity occurred, on average, at the same age as peak height velocity [33]. Knowing the maturity status is an important factor for training having into account that later maturing players had a significantly higher overuse injury incidence than their earlier maturing counterparts both in the year before peak height velocity and the year of peak height velocity [34]. Knowing the growth pattern should also be taken into account in the periodisation of training with a performance enhancement objective. For example [PERSON] et al. [35] demonstrate the importance of gains in lean body mass across later adolescence that support the ability to generate horizontal speed and predicted vertical power. Despite longitudinal studies that analyse changes between different years, there are hardly any studies that deal with changes between trimesters of the same year. For example [36] conclude that in children aged 5-10 years from September to April (season) is when most height is gained, while weight does not seem to change. These changes in height could be due to variables such as the light-dark cycle, temperature, or humidity among others. The study was conducted in Houston which has a temperate climate which may facilitate engagement in outdoor physical activity throughout the school year which causes weight gains to be small [36]. In the case of our study something similar could have occurred. The greatest increases in height occurred in the first trimester, perhaps due to the inertia of height growth during the summer. On the other hand, in our study we found that the greatest increases were in height. It is possible that height is the variable of all those measured that is most dependent on genetic factors.