Outdoor Exercise for People with Rheumatic Disease-Study Protocol and Baseline Characteristics of the OUTdoor Physical ACtivity Cohort (OUTPAC).

The OUTPAC cohort study evaluates the setup and implementation of a nationwide Danish initiative focused on the impact of structured outdoor physical activity (PA) on individuals with rheumatic diseases. This prospective cohort study includes more than 1600 participants, predominantly women (92%), with an average age of 65 years (range: 28-93). The cohort primarily consists of individuals with osteoarthritis (72%), rheumatoid arthritis (18%) and nonspecific lower back pain (13%). Volunteer instructors conducted interventions in outdoor settings, targeting strength, balance, physical capacity, mental health, and interaction with nature. Data collection involved questionnaires and physical tests in four primary outcome domains: quality of life, pain, physical function and activity, and mental health. Despite moderate pain (VAS mean: 48.3), high medication use (71%), and serious fatigue (54%), participants indicated having a good quality of life (EQ-5D-5L mean: 0.81) and average mental health (WHO-5 mean: 62.9). Baseline scores from physical tests showed results comparable to the general population aged 60-69 years. The OUTPAC project offers clinical insight into the implications of outdoor PA interventions on individuals with rheumatic disease while shedding light on the development and implementation of a large-scale nationwide outdoor PA intervention. Future analyses will examine short- and long-term changes and potential determinants.