## 5 Conclusions  
This study describes the MAT, CA, and sex-specific differences found in trunk muscle endurance through five field-based tests in a large sample of children and adolescents. In conclusion, there is a clear trend that trunk muscle endurance increases as participants become older or more mature. In relation to sex, boys presented higher performance in the SB and DEE tests, while girls performed better in the BS test. Furthermore, these sex-differences in performance were maintained when test scores were compared between peers of the same CA or MAT group. However, with respect to the CA and sex, differences were only found in the DEE test from the age of 14 onwards, where boys performed better than girls. Additionally, interactions were also found between sex and MAT, where boys classified as circa- and post-PHV performed better than girls in all tests. On the other hand, pre-pubertal boys and girls showed similar performance in almost all tests. Therefore, taking into account that physical fitness seems to be particularly sensitive to MAT, it is important to consider biological maturation when assessing physical fitness, rather than the CA, which is commonly used in international fitness batteries for children and adolescents. Despite these findings, this study has several methodological limitations that should be acknowledged. First, the estimation of biological maturation was based on a predictive equation using anthropometric variables rather than the gold standard method (hand-wrist radiographs). Future research should consider more precise methodologies for assessing lower limb length and biological maturity to reduce potential errors in classification. Additionally, sample distribution was not completely balanced across all age and maturation groups, which may have influenced some results. Future studies should aim to recruit more homogeneous and representative samples to minimize disparities. Addressing these aspects will contribute to refining the understanding of how maturation influences trunk endurance performance in children and adolescents. Conceptualization, M.T.M.-R. and F.J.R.-P.; methodology, M.T.M.-R. and F.J.R.-P.; formal analysis, M.T.M.-R.; investigation, P.S.d.B. and M.D.S.C. ; resources, P.S.d.B. ; data curation, M.T.M.-R. and F.J.R.-P.; writing--original draft preparation, M.T.M.-R.; writing--review and editing, M.T.M.-R. and F.J.R.-P.; visualization, P.S.d.B. and M.D.S.C. ; supervision, P.S.d.B. and M.D.S.C. ; project administration, P.S.d.B. ; funding acquisition, M.T.M.-R. and P.S.d.B. All authors have read and agreed to the published version of the manuscript. This work was carried out within the project \"Back pain prevention through Physical Education\" (DEP2010-21793) and supported initially by the Ministry of Science and Innovation, a subprogram of non-oriented basic research projects, and by the Ministry of Education, Culture and Sport (Spain) in the form of a predoctoral fellowship (FPU15/05200) awarded to [PERSON]. Additionally, [PERSON] was supported by a [PERSON] postdoctoral fellowship (111/MSJD/21) given by the Spanish Ministry of Universities and funded by the European Union-NextGenerationEU. The I SQUIOS Program is the result of the collaboration agreement signed between the regional Ministry of Education, Youth and Sports of the Region of Murcia through the General Directorate of Diversity Attention and Educational Quality, as well as the Sport and Musculoskeletal System Research Group (E0B5-07) of the University of Murcia since 2009-2010. [PERSON] was supported by a [PERSON] postdoctoral fellowship (UMU/R-1500/2021) given by the Spanish Ministry of Universities and funded by the European Union-NextGenerationEU. This study was conducted in accordance with the Declaration of Helsinki and approved by the Review Committee for Research Involving Human Subjects at the University of Murcia (Spain) (protocol code: 1920/2018). Informed Consent Statement: Informed consent was obtained from all subjects involved in this study.