The investigators have previously shown that youth with MS are very inactive, and that vigorous physical activity is associated with higher levels of well-being and lower MS disease activity in youth. Yet, no effective physical activity interventions have been developed for youth with MS to date. The investigators have taken input from youth with MS to create a Smartphone-based app (the ATOMIC - Active Teens with Multiple Sclerosis - App) that provides tailored physical activity information and coaching, provides tools to increase social connectedness, and promotes physical activity. This proposed research will therefore address the problem of inactivity in youth with MS by studying an intervention to increase physical activity.

Youth with MS have highly active disease and report high levels of fatigue and depression. As MS is a lifetime diagnosis, amelioration of these outcomes may have a sustained and important effect on the lives of these youth. Effective interventions oriented towards improving these outcomes are therefore imperative to develop and study. Importantly, recent work suggests that increased PA has the potential to improve brain tissue integrity, re-myelination, mental health outcomes, and quality of life in youth with MS. The investigators have demonstrated associations between lower levels of PA and higher levels of depressive symptoms and fatigue in these youth over time. Furthermore, the investigators have demonstrated an association between higher levels of PA and lower levels of disease activity in youth with MS. Importantly, preliminary work by the investigators shows that youth with MS have very low levels of PA. Increasing PA, therefore, has the potential to have both disease-modifying and psychosocial benefits in youth with MS.

The investigators have developed a user-driven app and program, the ATOMIC intervention, which addresses barriers to PA participation the investigators previously identified. The program provides youth with tools to increase goal setting, PA self-efficacy, and knowledge, and is embedded in a youth-focused app that is supported by health coaches. Notably, in preliminary work, the investigators have found the ATOMIC program to be acceptable to youth, and furthermore, that it was associated with a 31% increase in physical activity.

These strong preliminary results support moving forward with this proposed research, a multi-center randomized wait-list controlled trial (RCT) of the ATOMIC mobile app and coaching-based physical activity (PA) intervention in youth with multiple sclerosis (MS). The proposed study will examine the extent to which the intervention can change PA levels in this cohort. Additional goals will be to evaluate the effect of the ATOMIC intervention on behavioural change mediators, fitness, and psychosocial outcomes.

For this study, the investigators will recruit 56 youth with MS followed at three tertiary children's hospitals with large, established pediatric MS programs: The Hospital for Sick Children, Children's Hospital of Philadelphia, and the University of Alabama at Birmingham.

Inclusion Criteria:

1. Youth 11-21 years of age;
2. MS diagnosis or clinically isolated syndrome, as per revised McDonald diagnostic criteria and International Pediatric MS Study Group criteria prior to the age of 18;
3. Participating in less than three hours of structured physical activity per week.

Exclusion Criteria:

1. Have non-specific white matter abnormalities and metabolic or infectious etiologies for white matter abnormalities;
2. Do not speak and read English at a level needed to complete the questionnaires (4th grade level);
3. Have significant motor disability, classified as an Expanded Disability Status Scale (EDSS) ≥4;
4. Are at increased risk of cardiac or other complications of exercise testing (e.g. cardiac disease, diabetes), as determined by the pediatric neurologist or physician.

All participants will receive both the intervention and the attention-control condition and will not be informed of which of the procedures is the active intervention condition. The statistician, PI, and fitness testers, and physical activity data analysts will be blinded to the group assignment.