## 4 Discussion
### Relations Between Body Composition, Motor Competence and Physical Fitness Index  
Our results showed that FMR is more strongly and negatively related to MC and PFI than WC and BMI. Traditional anthropometric indicators, such as BMI and WC, measure body fat. However, the differences in BMI in overweight/obese young people are mainly explained by the fat mass [6]; on the other hand, in thinner children, the differences in BMI may be largely due to lean mass [22], including body water, smooth and skeletal muscle mass, and bones [23]. WC assesses central body fatness. Both, even if they are the most commonly used in relation to health, only consider the effect of fat on metabolism, ignoring other tissues, such as skeletal muscle [24]. Fat mass is known to provide important information about body fat gain and is associated with an increased risk of diseases such as obesity, cardiovascular disease, type 2 diabetes, and hypertension, among others [25]. Likewise, skeletal muscle provides information about physical performance [26], posture maintenance, and body movement in adults, children, and adolescents [27]. In addition, fat-free mass with a high amount of muscle mass has been shown to reduce the negative results of fat mass [24]. Figure 4: Histogram for (**A**) motor competence among FMRs for all children; (**B**) physical fitness among to FMRs for all children; (**C**) physical fitness among FMRs in boys. \(p\) values are from the \(t\)-test. Skeletal muscle constitutes approximately 40% of total body mass and plays a pivotal role in metabolic health, not only by regulating lipid and glucose metabolism [28] but also by producing myokines, which are partly dependent on skeletal muscle contraction [29]. As a contractile tissue, muscles facilitate neuromuscular activities such as motor skills, which are crucial for motor competence and physical fitness. Moreover, muscle function is understood not in isolation but within the framework of the bone-muscle-fat crosstalk, where the metabolic interactions among these tissues are interdependent, and mediated by autocrine, paracrine, and endocrine mechanisms [24]. Excess fat mass is known to affect bone metabolism both positively and negatively. On a positive note, adipose tissue acts as an endocrine regulator, secreting hormones and cytokines that can influence bone metabolism, particularly during early childhood. However, over time, this relationship weakens and reverses, with excess fat mass--especially when accompanied by adverse metabolic changes--contributing to negative effects on skeletal structure and strength, in addition to increasing the mechanical load on bones [30]. Multiple factors influence PFI, but one of the strongest is skeletal muscle function, not just because of the increased energy consumption, but also due to the amount of muscle mass that generates intensive crosstalk between organs and tissues, partly through the secretion of myokines. Some examples are (a) muscle-muscle crosstalk, explained by Musclin, LIF, IL-4, IL-6, IL-7, and IL-15 promoting muscle hypertrophy, and Myostatin which inhibits muscle hypertrophy; (b) muscle-brain crosstalk induced by Cathepsin B and Irisin, which cross the blood-brain barrier and stimulate BDNF production, leading to hippocampal neurogenesis, involved in the voluntary movement, and IL-6, stimulating appetite, therefore promoting energy gain; (c) muscle-adipose crosstalk, for lipolysis and for achieving decreased visceral fat mass through IL-6, with Irisin, meteorin-like, and IL-6 also having a role in the \"browning\" of white adipose tissue; (d) muscle-bone crosstalk, which is closely related with Decorin, IL-6, IGF-1, and FGF-2, which positively regulate bone formation; and finally, (e) muscle-gut crosstalk, involving Angiogenin, osteoprotegerrin, and IL-6 possess pancreatic \(\beta\)-cell protective actions, and IL-6, which increases insulin secretion from the intestine [31].