The proposed study is a parallel-group randomized controlled trial (RCT) designed to establish the behavioral mechanism underlying a 16-week physical activity (PA) behavioral intervention through its effects on the Capability-Opportunity-Motivation-Behavior (COM-B) constructs. The success of this study will provide robust evidence for supporting its refinement via refined COM-B constructs and Behavior Change Techniques to facilitate or maximize the intervention effects on PA levels, which will be examined in the subsequent clinical trial. Such an approach will involve an RCT that examines the effects of a 16-week remotely delivered, Internet-based PA behavioral intervention via video behavioral coaching and newsletter compared with a waitlist control (WLC) condition for yielding improvements in the COM-B constructs among persons newly diagnosed with multiple sclerosis (PNDwMS, disease duration ≤ 2 years). The investigators hypothesize that participants in the PA behavioral intervention condition will demonstrate greater improvements in the COM-B constructs than those in the WLC condition. The investigators plan to recruit 50 PNDwMS, with a distribution of 25 participants in the PA behavioral intervention and 25 participants in the WLC condition. The primary outcomes are the self-report COM-B constructs (listed in the Outcomes section), which will be remotely assessed at baseline and post-intervention through an online battery of questionnaires via REDCap.

Specific Aim 1: to establish mechanisms for the behavior change intervention using a 2-arm pilot randomized controlled trial (RCT) of the intervention compared with the waitlist control (WLC) among PNDwMS. The investigators hypothesize that the 16-week COM-B-based behavioral intervention delivered by one-on-one behavior coaching and newsletters will result in larger changes in COM-B constructs (i.e., knowledge, outcome expectations, goal, intention, behavioral regulation, self-efficacy, fatigue, social support, and social and environmental barriers) than in the WLC group.

Specific Aim 2: to examine change in self-reported physical activity levels in PNDwMS who received the COM-B-based intervention compared with those who are in the waitlist control condition. The investigators hypothesize that the 16-week COM-B-based behavioral intervention will result in larger changes in self-reported physical activity outcomes than in the WLC group.