BACKGROUND: The optimal moment to deliver digital prompts for reducing sedentary behavior (SB) is when individuals are susceptible to prolonged SB (vulnerability) and open to behavior change (opportunity). This study aims to examine both vulnerability and opportunity. METHODS: A 14-day ecological momentary assessment study was conducted with 105 older adults. Participants wore a Fitbit activity tracker and an ActivPAL accelerometer and used a smartphone-based ecological momentary assessment app. Sensor-triggered questionnaires were delivered via the app after 30 minutes of SB to capture participants' activity, physical, social, and temporal contexts; feelings of pain and fatigue; and willingness to interrupt SB. Descriptive statistics identified states of vulnerability, whereas linear mixed models examined when participants are willing to interrupt prolonged SB, shedding light on states of opportunity. RESULTS: Ecological momentary assessment data (n = 2580) showed that older adults were most vulnerable to prolonged SB while watching TV (36.3%) and using digital devices (14.7%). Prolonged SB predominantly occurred at home (91.2%), in the evening (46.4%), when feeling fatigue (52.2%), and when experiencing mild pain (21.1%). The willingness to interrupt SB varied by activity, physical and social context, perceived pain, and level of fatigue, but not by time of day. Specifically, participants were more willing to interrupt their SB when engaging in mentally active SB and when indoors, alone, or with close relatives. CONCLUSIONS: Contextual factors significantly influence older adults' vulnerability to prolonged SB and their willingness to interrupt it. This empirical information can guide the design of just-in-time adaptive interventions that deliver strategically timed prompts to reduce prolonged SB in older adults.

Department of Information Technology Faculty of Engineering and Architecture Ghent University Ghent Belgium

Department of Movement and Sports Sciences Faculty of Medicine and Health Sciences Ghent University Ghent Belgium

Faculty of Physical Education and Sport Charles University Prague Czechia

Faculty of Science University of Hradec Kralove Hradec Kralove Czechia

Research Foundation Flanders Brussels Belgium

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