To compare the efficacy of a sub-threshold aerobic exercise intervention provided with mobile health coaching (STEP-mhc) and a light stretching program (LS) to LS alone for 11-18 yo youth with persistent concussion symptoms.

This is a minimal risk randomized controlled trial whose aim is to test the efficacy of an exercise intervention (STEP) for youth 11-18 yo with persistent concussive symptoms (1-6 months inclusive). The investigators are utilizing an active control (light stretching). The primary outcome will be concussion symptoms measured with the Health-Behavior Inventory (HBI) in the first month after randomization. Long-term efficacy will be examined using standardized measures of quality of life and function over 6 months, and potential mediators will be explored including brain derived neurotrophic factor (BDNF), fear-avoidance and physical fitness. Physical activity will be measured with accelerometry to evaluate fidelity.

Inclusion Criteria:

• Sports-related concussion that occurred 1-6 months inclusive prior to the start of the study, and was diagnosed by a clinician trained in concussion management consistent with the Zurich definition of concussion ("a complex pathophysiological process affecting the brain, induced by biomechanical forces [that] results in a graded set of clinical symptoms that may or may not involve loss of consciousness")
• Persistent symptoms as defined by the presence of at least 2 concussive symptoms (from the Health Behavior Inventory, or HBI) at the time of recruitment which were not present prior to injury
• 11 to 18 years of age (inclusive) at time of recruitment

Exclusion Criteria:

• Non-English speaking youth or parents
• Unwilling or unable to travel to Seattle Children's Hospital or Seattle Children's Research Institute twice during the course of the study
• Cervical spine or vestibular issues requiring further targeted intervention
• Other injuries or medical conditions in addition to concussion which would preclude vigorous exercise
• Psychiatric hospitalization in the year prior to enrollment
• Significant cognitive deficits such as mental retardation or autism
• Lack of worsening of concussive symptoms with exertion
• Significant abnormalities on routine brain imaging.