## 4 Discussion  
The present study aimed to describe and analyse the effects of a 12-week football programme on body composition (fat mass, muscle mass and bone mass) in male children compared to a sedentary lifestyle, as well as the physical demands and loads required in training and matches in young football players. The main results regarding body composition indicated a significant increase in total BMD, BMD, and BMC of the legs in FG (U10 and U12). Higher whole-body values were found in CG-U12 for fat mass. FG  
Figure 1: Training load metrics for U10 and U12 players relative to competitive match play. Data are presented across positions: defenders (DFs), midfielders (MFs), and forwards (FWs). TD = total distance; distance Z5: distance zone 5; distance Z6: distance zone 6; VMAX = maximum speed; IACC = time interval between accelerations; VMAX\({}_{\text{ACC}}\) = average maximum speed acceleration; ACC\({}_{\text{MAX}}\) = maximum acceleration; TD\({}_{\text{ACC}}\) = average distance travelled in acceleration greater than 2.5 m-s\({}^{-2}\); HI acceleration = number of high-intensity accelerations; 1 = differences between DFs and MFs; 2 = differences from FWs. U10 and U12 had significantly increased VO\({}_{2}\) max values after 12 weeks of training. The physical demands and loads required in training and matches in FG differed according to the position, being higher for the U10 MFs. However, no significant differences between positions were obtained for U12 players. In addition, the load and physical demands were higher in training for U10 players than in matches. This variability was lower for U12 players. The results of the body composition testing together with the analysis of the load and physical demands of U10 and U12 football training in the present study are novel, as such a study has not been previously carried out. When analysing baseline body composition and fitness values, differences were observed between FG and CG, and these differences persisted or increased after 12 weeks of football training. Similar results were shown in the study by [PERSON] (2008) [29] in which the football group with at least 3 years of practice showed differences from the sedentary control group that only performed physical education classes, in terms of baseline values and after 5 and 10 months of football training. CG-U12 had higher fat mass values than FG-U12 before the intervention. However, these basal differences were not evident between CG-U10 and FG-U10. Furthermore, baseline VO\({}_{2}\) max values were higher in FG-U10 and FG-U12 than in CG-U10 and CG-U12. This difference was greater in the U12 category. This may be due to the length of time spent playing football, as the U12 players had been playing football for two years longer than the U10 players. This length of time playing sport may have resulted in a greater difference in energy expenditure and calorie consumption compared to CG-U12, which may have led to greater differences in fat mass levels [30]. Football is an aerobic sport that leads to improvement in cardiorespiratory fitness from childhood to adolescence and can lead to improvement in VO\({}_{2}\) max compared to CG [31]. Additionally, CMJ values were higher in FG-U12 before and after the intervention than in CG-U12. Football is a combination of jumping, sprinting, changing direction, shooting, and passing [10]. These physical activities mainly use the skeletal muscles of the lower extremities, resulting in greater strength and power compared to CG. Therefore, previous football experience is a factor to consider when analysing body composition and physical fitness in U10 and U12 children. In relation to the values obtained in the bone development of U10 and U12 children, we found higher levels of total BMC and BMD and in the legs of both groups (FG-U10 and FG-U12) after the 12 weeks of training. This is due to the fact that a sport such as football is associated with high-impact exercise, as it involves sprints, jumps, and movements that require abrupt changes in the centre of mass [32].