The ACDSi 2023–24 study protocol: tracking 50 years of physical fitness trends and their determinants in Slovenian children and youthJurak, Gregor (Avtor) Leskošek, Bojan (Avtor) Besal, Sara (Avtor) Meh, Kaja (Avtor) Premelč, Jerneja (Avtor) Golja, Petra (Avtor) Markelj, Neja (Avtor) Kereži, Urška (Avtor) Robič Pikel, Tatjana (Avtor) Zdešar Kotnik, Katja (Avtor) Potočnik, Žan Luca (Avtor) Rojko, Tjaša (Avtor) Bezjak, Nika (Avtor) Sember, Vedrana (Avtor) Kovač, Marjeta (Avtor) Starc, Gregor (Avtor) sportanthropometryhealthmotor abilitiesmotor developmentphysical activityphysical fitnesssecular trendssomatic developmentThe Analysis of Children’s Development in Slovenia (ACDSi) is a unique decennial national study that has been monitoring somatic and motor development in Slovenian children and adolescents for more than five decades. It offers unparalleled insight into long-term secular trends in physical fitness and related correlates and determinants. This paper presents the study design and updated methodology of the ACDSi 2023/24 edition, ensuring continuity with previous editions and providing a robust foundation for future comparisons. The ACDSi 2023/24 edition employed a cross-sectional, sentinel-site design, building upon the established methodologies of previous editions. We successfully recruited 3,853 children and adolescents aged 6–18 years, representing 1.35 % of the total population in this age range. The robust research design employed a two-stage cluster sampling technique to ensure national representativeness across various settlement types. Comprehensive data collection included 12 fitness tests, over 25 anthropometric measurements and questionnaires. Surveys were administered online to adolescents (11–18 years) and in paper format to parents of children (6–14 years), capturing information on 24-h movement behavior, commuting habits, sport activity, motivation, self-concept, family environment, socioeconomic background, and nutrition. The methodology establishes a vital database foundation for the subsequent decennial edition, planned for 2033/34. Adherence to this methodology is crucial, distinguishing the ACDSi for its holistic approach to characterizing physical fitness and its determinants. This rigor allows researchers not only to track secular trends in fitness every decade but also to simultaneously observe shifts in the complex determinants affecting fitness trends in children and adolescents. The ACDSi 2023/24 study preserves the methodological continuity of earlier editions by generating harmonized datasets, while simultaneously broadening its scope through the inclusion of additional indicators that capture the influence of evolving social and technological environments on children’s and adolescents’ lifestyles.20262026-02-10 13:18:45Članek v reviji179312UDK: 796.01ISSN pri članku: 2296-2565DOI: 10.3389/fpubh.2025.1734046COBISS_ID: 265813251sl