Sedentary time elimination with periodic activity snacks (STEPS): protocol for a codesigned feasibility trial in people with severe mental illness.

introductionPeople with severe mental illness (SMI) experience substantially reduced life expectancy, largely due to cardiovascular disease (CVD). Sedentary behaviour is a major risk factor for CVD, and people with SMI spend significantly more time sedentary and are less physically active than the general population. While interventions to increase physical activity have focused on structured exercise, these are not accessible or acceptable to many consumers. The sedentary time elimination with periodic activity snacks (STEPS) study aims to test and evaluate the feasibility of a codesigned novel intervention using 'activity snacks' to interrupt sedentary behaviour in people with SMI. METHODS AND ANALYSIS: The STEPS study is a single-arm feasibility trial. We will recruit 20 participants with SMI to complete a 6-week intervention. The intervention will include the use of the STEPS smartphone app to prompt short bouts of movement across the day, the type of which are personalised to the individual. Feasibility will be assessed through recruitment and retention rates, adherence to the intervention and acceptability measured via qualitative interviews. Secondary outcomes include changes in sedentary time, physical activity, mood and quality of life, with pre-post variables analysed via either a t-test or Mann's Whitney U test, dependent on the normality of the data. Effect sizes for secondary outcomes will be calculated as Cohen's ETHICS AND DISSEMINATION: This trial has received ethical clearance from the Metro South Human Research Ethics Committee (HREC/2025/QMS/117193) and was prospectively registered with the Australia New Zealand Clinical Trials Registry (ACTRN12625000265471p). Results from this trial will be published in a peer-reviewed journal. TRIAL REGISTRATION NUMBER: Australia New Zealand Clinical Trials Registry (ACTRN12625000265471p).