Development of a single-item physical activity intention measure for adults with intellectual disabilities: Evidence of validity and reliability.

Background: Physical activity (PA) is important for overall health and well-being, but adults with intellectual disabilities often struggle to achieve adequate levels of PA. Therefore, it is necessary to understand their PA behaviour.

Objective: To develop a Single-Item Physical Activity Intention Measure (SPAIM) to assess PA intentions of adults with intellectual disabilities and preliminarily analyse its psychometric properties in terms of validity and reliability.

Methods: The study had three phases: developing the SPAIM, assessing its validity and reliability evidence, and conducting a cross-sectional survey to analyse the relationship between PA intentions (measured by SPAIM) and PA levels. Participants were recruited between September 2016 and August 2017 from diverse settings in Oxford, UK, to ensure representation across the intellectual disability spectrum.

Results: There were 82 participants aged 20-68 (mean age 36 ± 13) years who had mild-profound intellectual disabilities. The study provided evidence of content- and response processes-related validity to ensure respondents' understanding. Additionally, test-criterion evidence was provided, showing a predictive correlation between PA intention and sedentary hours/day (12 % of the explained variability). However, there was no correlation between PA intention and PA minutes/week. The study also provided adequate test-retest evidence (r = 0.78).

Conclusions: SPAIM may be a valuable tool for measuring PA intention in adults with intellectual disabilities. Modifying PA intentions could prove crucial in reducing sedentary behaviour and improving the health outcomes of this population. Future research and application of SPAIM in varied contexts will deepen our understanding of PA intentions and explore its predictive characteristics.