The proposed ATOMIC intervention consists of four primary components: (a) one-on-one chats with a PA coach, (b) informational posts, (c) PA self-monitoring through an activity tracker and (d) educational modules regarding different aspects of becoming PA delivered through our MS specific PA app.

(a) PA Coach, One-on-one Chats. Coaches will conduct three one-on-one phone or video conferencing sessions and will also conduct a text messaging session once per week to reinforce and follow-up on the one-on-one phone sessions during the course of the intervention. (b) PA-Self Monitoring. An activity tracker (Fitbit Charge 2) will be used to allow participants to monitor their activity behaviors (e.g. step counts). (c) PA Information. In addition the coach will send weekly tailored PA informational posts/messages aligned with Social Cognitive Theory (SCT) to all participants. (d) Educational Modules. MS specific web-based interactive PA modules will be provided to patients.

Determined by calculating accrual rates, drop-out rates, adherence rates with intervention, adverse event rates, proportion of completed study questionnaires, accelerometry and proportion of complete data extraction from ATOMIC app

Utility will be addressed through semi-structured interviews that includes both open and closed ended questions. The interviews will be used to elicit the perceptions and preferences of the participants surrounding the use and method of delivery of the PA coaching and program content (e.g. phone, text messaging, posts to app); the usefulness, relevance, and applicability of the ATOMIC program content; as well as facilitators and barriers to use of the program. Semi-structured interviews will be conducted at baseline, six weeks, and post-intervention (twelve weeks). To supplement the interview data, utility will also be captured by counting the number of log-ins to the app and the number of completed coaching sessions and modules.

Utility will be addressed through semi-structured interviews that includes both open and closed ended questions. The interviews will be used to elicit the perceptions and preferences of the participants surrounding the use and method of delivery of the PA coaching and program content (e.g. phone, text messaging, posts to app); the usefulness, relevance, and applicability of the ATOMIC program content; as well as facilitators and barriers to use of the program. Semi-structured interviews will be conducted at baseline, six weeks, and post-intervention (twelve weeks). To supplement the interview data, utility will also be captured by counting the number of log-ins to the app and the number of completed coaching sessions and modules.

Utility will be addressed through semi-structured interviews that includes both open and closed ended questions. The interviews will be used to elicit the perceptions and preferences of the participants surrounding the use and method of delivery of the PA coaching and program content (e.g. phone, text messaging, posts to app); the usefulness, relevance, and applicability of the ATOMIC program content; as well as facilitators and barriers to use of the program. Semi-structured interviews will be conducted at baseline, six weeks, and post-intervention (twelve weeks). To supplement the interview data, utility will also be captured by counting the number of log-ins to the app and the number of completed coaching sessions and modules.

The effect of our PA intervention on MVPA level and the mediators and moderators of SCT (e.g. self efficacy) will be determined by calculating the change in time spent in MVPA and mediators and moderators of SCT at the completion of the intervention.