People with severe mental illness (SMI), such as schizophrenia, bipolar disorder, or major depressive disorder, experience a life expectancy reduction of 10-20 years, largely due to chronic conditions like cardiovascular disease and type 2 diabetes. These individuals are less physically active and spend more time in sedentary behaviours (SB), which are associated with increased mortality, poor metabolic health, worse cognition, and lower quality of life. Traditional studies often rely on self-reported SB, which is unreliable, whereas objective measures like accelerometry provide more accurate data. Understanding SB patterns is essential for designing effective interventions to reduce prolonged sitting and improve health outcomes in this population.

This study aimed to describe daily SB patterns in outpatients with SMI and evaluate the feasibility of the "Sit Less, Move More" program integrated into routine care at Osonament, a community mental health center in Catalonia. The pragmatic intervention lasted 16 weeks and included two weekly contacts: face-to-face sessions for planning walking routes and strategies to increase activity, and telephone follow-ups to reinforce goals. Participants were grouped by affinity and residence to encourage autonomous walks, and the program was embedded within individual recovery plans. Ethical approval was obtained, and informed consent was secured.

Sedentary behaviour and physical activity were measured using the ActivPAL™ accelerometer, worn continuously for seven days at baseline and post-intervention. The device provided detailed data on sitting time, standing, light activity, and moderate-to-vigorous activity, as well as SB bouts categorized by duration. Self-perceived health was assessed using a visual analogue scale. From 412 potential candidates, 231 were invited, 60 enrolled, and 53 completed the program. Dropouts were due to psychiatric relapse, hospitalization, discharge, relocation, or loss of interest.

The study demonstrates the feasibility of integrating SB reduction strategies into community mental health services and highlights the importance of objective SB measurement. It provides a foundation for developing tailored interventions that break up prolonged sitting and promote movement among people with SMI, addressing a critical gap in improving physical health and reducing premature mortality in this vulnerable population.

Inclusion Criteria:

• Being an active user of Osonament
• Having a clinical diagnosis of a severe mental illness (e.g., schizophrenia, bipolar disorder, etc) in a stable plhase of the illness

Exclusion Criteria:

• Actively using toxics substances
• Had a predominant diagnosis other than a sever mental disorder
• Presented with medical contraindication that prevented safe participation in physical activity