## 1 Introduction  
Physical fitness encompasses a collection of measurable attributes associated with the capacity to engage in physical activity, serving as a significant indicator of overall health [1]. It can be conceptualized as a comprehensive measure of several body functions, including skeletal-muscular, cardiorespiratory, circulatory, psychoneurological, and endocrine-metabolic systems, all of which are crucial for daily physical exertion [2]. Furthermore, regular physical activity yields numerous health benefits, positively impacting aspects such as self-esteem and social relationships [2, 3, 4, 5]. Consequently, physical fitness closely correlates with overall health, as individuals with higher fitness levels generally exhibit better health statuses [6, 7]. Adolescence, spanning ages 11 to 19 according to the World Health Organization (WHO) (with early adolescence from 11 to 13 years, middle adolescence from 14 to 16 years, and late adolescence from 17 to 19 years), represents a pivotal period for cultivating healthy lifestyle habits, including engagement in physical activity during leisure time [8, 9]. However, adolescents often encounter behaviors posing health risks, such as alcohol and tobacco consumption [10]. These formative years are crucial because behaviors established during this time significantly impact long-term health outcomes [11]. Notably, adolescents frequently abandon positive habits like sports or exercise, contributing to physical inactivity and declining fitness levels [12], which, in turn, increase the risk of various diseases like obesity and diabetes [13]. Recent studies have highlighted a marked decline in physical fitness levels among children and adolescents [14; 15; 16; 17; 18]. Additionally, factors such as insufficient physical activity (less than the 60 min per day recommended by the WHO), consumption of high-calorie diets, and sedentary behaviors (including extended periods of sitting, excessive use of technology, and reliance on motorized transportation) negatively impact health and quality of life. The PASOS (Physical Activity, Sedentarism, and Obesity in Spanish Youth) study found that only 36.7% of children and adolescents adhere to WHO exercise guidelines [19]. Furthermore, the study indicated that inactivity rates were higher in girls than boys (70.1% vs. 56.1%) and in adolescents compared to children (69.9% vs. 56.1%). As a result, epidemiologists suggest that physical fitness could be essential in combating overweight and obesity in this demographic [20]. This is particularly concerning as a physically active lifestyle is known to positively influence brain structure and function during childhood [21; 22; 23]. Beyond the physiological benefits of exercise [24], its positive effects on mental health [25], bone health [26], cognitive abilities [27; 28], and children's learning processes have been extensively researched. Most scientific evidence on cross-sectional changes in adolescent fitness has centered on analyzing aerobic capacity [29]. This approach can erroneously convey the idea that other capabilities like speed, strength, or agility are not important, thus biasing the impression that only aerobic fitness is key for health and locomotive function maintenance [30]. In fact, abilities such as speed or agility are strongly correlated with bone mineral density and the accumulation of bone mass during the lifespan [2; 7]. Moreover, a significant portion of physical activities undertaken by children and adolescents consists of high-intensity exercises, necessitating anaerobic energy production [31]. In support of this, [PERSON] et al. [32] observed that children aged 6 to 10 years often participate in activities characterized by short bursts of intense effort. Consequently, variations in anaerobic capacity, including abilities like changing direction or performing interval sprints, can be indicative of shifts in these routine physical activities.