Urban RCT participants were healthier than non-participants or rural women.

OBJECTIVE To investigate sociodemographic characteristics and physical and mental health indicators between participants and nonparticipants of a large-scale 2-year exercise RCT including noninvited women living in nearby rural area in Finland. STUDY DESIGN AND SETTING From a previous OSTPRE study cohort, 914 women (aged 72-84) participated in Kuopio Fall Prevention Study in 2016-2019. The participants were compared to non-participants (n = 4,536) and noninvited OSTPRE women (n = 7,119) living outside the urban recruitment area. RESULTS Participants were younger (P< 0.001) with higher education (P< 0.001) and had more often regular hobbies (P< 0.001) and physical exercising (P< 0.001) than nonparticipants or noninvited. They reported better functional capability (P< 0.001), mental (P< 0.001) and subjective health (P< 0.001), lower number of medications (P< 0.001), less fear of falls (P< 0.001), but more frequent falls (P= 0.002) and more often musculoskeletal diseases (P= 0.006).  Participants also showed better functional capacity in the clinical measurements. In register analysis, urban-rural differences in the prevalence of diseases were detected. CONCLUSION In population-based exercise interventions, participants are more likely to be better off in respect to physical and mental wellbeing, functional capability and sociodemographic status. Recruiting participants only from cities increases unavoidable selection bias due to urban-rural differences which should be noticed when interpreting and generalizing RCT results. TRIAL REGISTRATION NUMBER ClinicalTrials.gov identifier NCT02665169.