## 4 Discussion
Perhaps for the reasons mentioned above, sex differences according to CA appear in this study, only in a dynamic test (DEE test) where the application of strength endurance plays a key role during the execution of these tasks rather than endurance per se, since it coincides with the onset of the pubertal growth spurt in boys and the development of strength discussed previously. Considering the MAT, sex differences were more pronounced in all tests for participants in circa-PHV, whether in static or dynamic tests, providing better insights into the results than when using only the CA for analysis. Finally, no significant interaction was found between sex and pre-PHV. These results of performance in pre-PHV students are unsurprising and in agreement with previous studies analyzing not only trunk muscle endurance but also upper and lower extremity strength or power [30,43]. Maturational sex differences before the adolescent spurt do not differ significantly in body height, body mass, girth, bone width or skinfold thickness, and physical fitness [27,28,30]. The results of maturation should be interpreted with caution, considering the methods used to estimate PHV. Skeletal age via a hand-wrist radiograph is considered the \"gold standard\", but it has clear disadvantages. These include significant radiation exposure for participants, invasiveness, high costs, and time requirements, as well as the need for a high level of expertise to administer [55]. Although [PERSON] et al. 's [33] predictive equation is widely used in the literature because it is non-invasive, time efficient, and practical for child-youth contexts, other equations may be more appropriate for estimating maturity offset, such as that of [PERSON] et al. [56] or that of [PERSON] et al. [57] (only for boys), as well as the [PERSON] and [PERSON] equation [58] used to estimate the percentage of the final estimated adult stature attainment (%EASA). Regardless of the method employed, from a practical standpoint, it is recommended that anthropometric measurements and subsequent maturation estimates be performed at least three times per year on a routine and consistent basis. This will enable sports professionals and PE teachers to identify the onset and cessation of PHV and therefore prescribe training loads based on maturation status [55]. Based on the results obtained, youth fitness specialists (i.e., researchers, physical therapists, PE teachers, and strength and conditioning coaches) should be aware of the effects of the CA and MAT when assessing trunk muscle endurance, in particular, and physical fitness, in general, in children and adolescents. In turn, despite taking into account that previous studies have shown that the reduced endurance of trunk muscles and the endurance imbalance (asymmetry) between trunk muscle groups are related to the higher prevalence and intensity of BP in children and adolescents [18,19,22,59,60],and that the improvement in the endurance of the flexor and extensor muscles of the trunk is associated with the reduction in LBP in adolescents [61; 62], most health-related physical condition test batteries for children and adolescents only include dynamic trunk flexor measures, that is, sit-up or curl-up tests (e.g., Eurofit, Connecticut Physical Fitness Assessment, FITNESSGRAM(r)/ACTIVITYGRAM(r), ALPHA-Fitness test battery). That is why we decided to include five different field-based tests, both dynamic and static, as well as in the sagittal and frontal planes, covering the different muscle groups that participate in trunk movements, since there is no single test to measure trunk muscle endurance. Each test has its advantages and limitations, and the choice depends on many factors, such as the context in which the test will be applied, the characteristics and needs of the participants, and the capability of the trunk muscles to be measured [63].