Research on 100-year trends in physical fitness among youth in the territory of the former Czechoslovakia: Design and methods of the project Background: In 2023, exactly 100 years will pass since the first nationwide testing of the physical fitness level among youth in the territory of the former Czechoslovakia. In the context of this completely unique and unrepeatable opportunity, there is the possibility of finding objectively measured secular trends in physical fitness in our countries. Objective: The main objective of the project is, based on the historical methods known to us, to implement a comprehensive objective testing of the physical fitness level among 11-19-year-old youth in the territory of the former Czechoslovakia with an interval of 100 years and to compare the results with the original study. The purpose of this article is to present the research design including detailed methods. Methods: The methods of repeated testing of the physical fitness level are reduced, however, it is prepared in accordance with the earlier historical methods. The study will be carried out on a research group of 11-19-year-old individuals selected from the expected final number of 20 participating schools (15 Czech and 5 Slovak schools). The schools will be selected by a stratified selection from the preserved historical list of originally participating schools. The testing will include a total of 3 somatic measurements (body height, sitting height and body weight) and 5 motor tests (standing broad jump, standing small ball throw, pull-ups, speed run and endurance run). Data collection is planned for a period exactly corresponding to 100 years from the initial research (March to April 2023). Results: Among other things, the results will contribute to the accurate determination of evidence-based long-term trends in physical fitness in our youth population. Conclusions: The presented design of the study introduces, in addition to the rationale, goals, expected outputs, detailed research methods, which are aimed at finding objectively measured 100-year trends in physical fitness among youth in our territory. At the same time, the research project will contribute to increasing the awareness of experts and the public about the given area.