BACKGROUND: Although regular physical activity (PA) mitigates the risk for cardiovascular disease (CVD) during midlife, existing PA interventions are minimally effective. Harnessing social influences in daily life shows promise: digital micro-interventions could effectively engage these influences on PA and require testing. PURPOSE: This feasibility study employed ecological momentary assessment with embedded micro-randomization to activate two types of social influences (ie, comparison, support; NCT04711512). METHODS: Midlife adults (n = 30, MAge 51, MBMI 31.5 kg/m2, 43% racial/ethnic minority) with >= 1 CVD risk conditions completed four mobile surveys per day for 7 days while wearing PA monitors. After 3 days of observation, participants were randomized at each survey to receive 1 of 3 comparison micro-interventions (days 4 to 5) or 1 of 3 support micro-interventions (days 6 to 7). Outcomes were indicators of feasibility (eg, completion rate), acceptability (eg, narrative feedback), and potential micro-intervention effects (on motivation and steps within-person). RESULTS: Feasibility and acceptability targets were met (eg, 93% completion); ratings of micro-intervention helpfulness varied by intervention type and predicted PA motivation and behavior within-person (srs 0.16, 0.27). Participants liked the approach and were open to ongoing micro-intervention exposure. Within-person, PA motivation and behavior increased from baseline in response to specific micro-interventions (srs 0.23, 0.13), though responses were variable. CONCLUSIONS: Experimental manipulation of social influences in daily life is feasible and acceptable to midlife adults and shows potential effects on PA motivation and behavior. Findings support larger-scale testing of this approach to inform a digital, socially focused PA intervention for midlife adults.

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