## 4 Discussion
Additionally, physiological mechanisms related to fatigue resistance, such as the activation of the motor neuron pool, synaptic inputs from metabolically sensitive afferent fibers, muscle perfusion, skeletal muscle metabolism, and fiber type properties, could further explain the observed differences in endurance between males and females [48]. These mechanisms highlight the complex interplay of anatomical, physiological, and neuromuscular factors contributing to sex differences in muscle endurance and fatigue resistance. In contrast, the BS test presents a notable exception to this general pattern. Several reasons have been proposed to explain this sex-related difference observed in the BS test: First, girls usually have greater lumbar lordosis than boys. Previous studies have shown that the lever arm length for the erector spinae muscles is higher with an increased lumbar lordosis [49], and the functional consequence of longer lever arm lengths is that less force is needed to create a certain external torque, such as lifting [50]. Second, females' erector spinae muscles have been shown to contain a greater proportion of slow-twitch muscle fibers than those of males [8]. The metabolic and physiological characteristics of slow-twitch (type I) muscle fibers provide them with superior oxidative capacity and the ability to maintain isometric contractions with more efficient tension maintenance compared to type II fibers [51]. Consequently, contractions predominantly supported by type I fibers are associated with a reduced rate of accumulation of metabolic by-products which have been linked to the onset of fatigue [52], thereby facilitating the endurance of this musculature in girls compared to boys. On the other hand, when test scores were compared between peers of the same CA or MAT group, these sex differences in performance were maintained. However, with respect to the CA and sex, differences were only found between boys and girls in the DEE test and from the age of 14 onwards, where boys performed better than girls. In addition, as hypothesized, interactions were also found between sex and MAT, where boys classified as circa- and post-PHV performed better than girls in all tests. Although these significant sex-specific differences have been observed in studies involving participants at all ages [15; 16; 23; 24; 25; 42], the marked acceleration of strength development during the growth spurt of male adolescents sets and magnifies these differences by males and females [27; 53]. As shown in previous research, during the onset of puberty, the responses in estradiol and testosterone levels are different in males and females [54], so this divergence could contribute to differences in physical fitness between sexes that are becoming increasingly evident during puberty, as seen in this study. For example, increases in insulin-like growth factor 1 hormone (IGF\(-\)1) and growth hormone foment protein synthesis in both males and females, but the additional anabolic effect of increased testosterone, means that boys experience greater formation and development of fast-twitch muscle fibers and much greater gains in muscle mass during puberty than girls [31,54]. In addition, increased testosterone in males promotes a significant rise in the growth of bone, height, and muscle mass [12,27,28,53], while increased estradiol in females promotes an increase in body fat and stimulates ovulation and breast development [16,30,53]. Therefore, the sex-related differences found in trunk endurance measures could be due to these physical changes associated with puberty, leading to greater performance in boys compared to girls during circa- and post-PHV [23,27,29]. The results of our study coincide with previous research, where it has been shown that females tend to perform worse during PHV on tasks that require weight-bearing or strength [29], as is the case of the tests used in the present study. In contrast, males generally perform better during PHV, especially in activities that require strength, speed, or power [27,29,30,31,53].