Project Summary/Abstract Older women are disproportionately impacted by a range of chronic diseases and conditions, such as cognitive impairments, that can be alleviated by regular physical activity (PA), including walking; yet they are the most inactive segment of the US population. While the majority of older women use smartphones, few remotely delivered PA interventions have been developed specifically to address this age + gender PA disparity, and few such interventions leverage built and social environmental factors that can increase daily PA. Using a dynamically based socio-ecological model of health behavior change, this study’s primary aim is to enhance the efficacy of a “light-touch”, remotely delivered PA educational intervention for older women by testing the added impacts of an innovative, remotely delivered citizen science program to identify and address local environmental barriers to walking and other PA (called Our Voice TM [OV]). The “light-touch” PA educational program has been delivered since 2015 to >23,000 women >70 years that have been participating in the U.S.-wide Women’s Health Initiative Strong and Healthy (WHISH) pragmatic PA trial (U01HL122280-03). WHISH, which used a pragmatic, ‘opt-out’ consent enrollment process, includes the broadest range of PA, physical function levels, and geographic regions found in PA intervention studies to date. The original WHISH educational program has resulted in significant but generally modest PA increas- es in the overall WHISH population, and surveys indicate that many WHISH participants could benefit strongly from further intervention. An enriched version of the original remote program (called enCore) will serve as the enCore Alone arm in the proposed trial. To address local built and social environmental PA barriers that many WHISH women describe, the mobile app plus web based OV program will be added to enCore (enCore+OV arm). We will randomize a total of 300 WHISH intervention women (mean age=85 yrs.) with smartphones who are insufficiently active to the two arms. We hypothesize that women receiving enCore+ OV will show higher pedometer-measured 12-month PA levels than women receiving enCore Alone. Additional questions include changes in cognitive function and sedentary behavior; exploring a theo- retically derived multi-level set of putative mediators (e.g., social cohesion, rated neighborhood walkability, community wayfinding) and baseline moderators of program success (e.g., race/ethnicity, family support, geographic region); and exploring the relative costs of the two programs for PA change. The study will add important information on the benefits and trade-offs of combining these remotely delivered, practical, and potentially scalable behavioral health delivery approaches for this fast-expanding demographic group.