Concussion in youth is increasingly being recognized as a national health concern, affecting more than one million youth each year. As many as 30% of youth with concussion experience persistent and disabling concussive symptoms lasting more than one month, defined as Persistent Post-Concussive Symptoms (PPCS). PPCS interferes with social relationships and adversely impacts physical and academic function. Previous recommendations for treating PPCS focused on cognitive and physical rest, but in 2016 guidelines shifted based on new research suggesting the benefit of rehabilitative exercise. The rationale behind the use of exercise is that gradual increases in the intensity of physical activity facilitates return to full function. Rehabilitative exercise has since become one of the most common approaches to treating youth with PPCS. However, most programs require weekly centralized visits with a concussion specialist, creating barriers to care. To bridge this gap, we developed an mHealth approach to treat PPCS, utilizing physical activity trackers (Fitbits) and weekly video conferences with trained research staff to eliminate the need for frequent clinical visits. We conducted a series of pilot studies with this approach, finding excellent feasibility, acceptability, and evidence for more rapid declines in concussive symptoms compared to controls. We also found preliminary evidence that mechanisms behind this intervention may stem from both physiologic processes due to increased moderate-to-vigorous physical activity (MVPA) and psychologic processes such as reducing fear- avoidance of concussive symptoms. We thus propose a fully powered, randomized controlled trial (RCT) to rigorously examine the ?Mobile Subthreshold Exercise Program? (M-STEP) for youth with PPCS. We will recruit n=250 youth ages 11-18 years with PPCS lasting 1 to 6 months, and randomize them to 6 weeks of either usual care plus M-STEP (intervention) or usual care plus stretching (control). Youth in M-STEP will be coached through gradual increases in intensity and duration of rehabilitative exercise via weekly video conferencing (Zoom) and will use Fitbits to guide their progress. Youth in the stretching group will act as attention controls, with equivalent study contacts. All youth will complete research-grade measures of MVPA using actigraphy and standardized surveys at multiple time points. Youth will be followed for a total of 6 months. We will use mixed effects models to examine differences between experimental groups in the primary outcome, trajectory of concussive symptoms, and the secondary outcome, changes over time in health-related quality of life, on an intention-to-treat basis. The proposed transdisciplinary R01 RCT will provide definitive information regarding the efficacy of M-STEP for youth with PPCS, examine potential mediators including MVPA and fear-avoidance, and explore the impact of biopsychosocial factors (patient sex and parental protective behaviors).