["Journal of Intellectual Disability Research, EarlyView. ", "\nABSTRACT\n\nBackground\nResearch has shown significant underdiagnoses in adults with intellectual disabilities, highlighting the need for objective health measurements to prevent overshadowing. Physical fitness, crucial for health and functioning, is consistently poor in this population, increasing the risk of adverse outcomes. Fitness tests are promising but to date lack specific reference values from large datasets for broader applicability. This study examines the feasibility of four physical fitness tests in two large European cohort studies, IDS‐TILDA in Ireland and the HA‐ID study in the Netherlands, to address barriers and improve inclusion in research and care.\n\n\nMethod\nThis study used data from IDS‐TILDA (40+ years) and HA‐ID (60+ years) cohort studies. Static balance was measured by the capability of maintaining three stances; muscle strength was measured as grip strength; muscular endurance was measured with the Five‐Times Chair Stand; and cardiorespiratory fitness was measured with the Two‐Minute Step Test. Reasons for non‐compliance were documented. The feasibility of these tests and reasons for non‐successful performance were described for the total group and across participant characteristics.\n\n\nResults\nThe IDS‐TILDA sample was younger than the HA‐ID sample, with more participants being independent in mobility. All fitness tests showed moderate to good feasibility, except the Full‐Tandem stance in the IDS‐TILDA sample. Lower feasibility was found in adults with severe and profound intellectual disabilities, walking aids and wheelchair users. Key barriers for participation were physical limitations and difficulties understanding the task.\n\n\nConclusions\nThis study showed overall moderate to good feasibility. Physical limitations and difficulties understanding the task were important barriers, emphasising the need for the person‐centred approach taken by both studies. Feasibility for including standardised physical fitness assessments, with a person‐centred approach, in large cohort studies is shown. Including physical fitness assessment is recommended to allow for comparability and combining of data for more knowledge on physical fitness in adults with intellectual disabilities.\n\n"]