More than one in five adult Latinos in the United States has arthritis and Latinos experience greater joint pain and limitations than non-Latino Whites. Physical activity (PA) improves arthritis symptoms, yet Latinos are less likely to engage in any PA (50%) than non-Latino Whites (65%). Camine con Gusto (CCG), the Arthritis Foundation’s evidence-based 6-week Spanish-language version of Walk With Ease, is one of few PA interventions tested in Latinos. CCG was shown in a pre-post study to improve outcomes in 233 Latinos with arthritis. The Osteoarthritis Action Alliance developed an online portal for CCG with enhancements including weekly emails/texts, tracking/goal setting, and video/audio content based on feedback from Latinos and community-based organizations (CBOs) that offer CCG. Participants expressed a need for additional, more resource-intensive components to CCG that have yet to be implemented or tested. The overall goal of this R34 is to plan an assessor blinded community-based randomized optimization trial to test recommended additional components for CCG PA initiation and maintenance. To evaluate which additional components contribute meaningfully to increasing PA, we will use the Multiphase Optimization Strategy (MOST), an approach to developing highly efficient and scalable behavioral interventions that include only the most potent and cost- effective components. Using MOST, we will: 1) Determine the set of components (pedometer, CCG promotora, and maintenance promotora) when added to CCG that meaningfully increase daily steps (primary outcome) immediately post-CCG and 6- and 12-months post-CCG; 2) Evaluate the set of components, when added to CCG, that meaningfully improve secondary outcomes (i.e., arthritis symptoms, sleep quality) post-CCG and at 6- and 12-months; and 3) Evaluate the cost-effectiveness of the intervention components for scalability in CBOs serving Latinos with arthritis. We have worked closely with our Community Advisory Board (CAB) on this R34 proposal and will work with them on all aspects of the planning grant and proposed trial. Results from each aim will inform the decision on which set of components has the greatest impact on PA initiation and maintenance for the best cost. In this planning period, we will: 1) Use CAB input and behavioral theory to finalize the scientific details for the study; this will include refining CCG portal enhancements and additional intervention components (i.e., pedometer and promotoras), finalizing outcome measures, collection methods, the research and cost-effectiveness protocols and statistical analysis plan. 2) Use CAB input to develop the materials and application for the proposed trial; this will include recruitment, retention, and adherence materials and approaches; manual of operating procedures; data management plan; training manuals, and fidelity assessment plans; data safety and monitoring plan; and a detailed project timeline and budget for the future clinical trial. T...