Adolescents with concussion will be randomly assigned to a treatment group where they will receive sub-threshold exercise, or assigned to a placebo group where they will receive structured stretching exercises. Both groups will receive standard medical coverage with regular clinic visits. All participants will record symptoms daily on a dedicated web site. All participants will be evaluated at time 1 with (1) structured physical exam, and (2) structured exercise stress test which is terminated when there is symptom exacerbation. The primary outcome measure is time to recovery where recovery is defined as (1) asymptomatic for two consecutive days, and (2) ability to exercise to exhaustion without exacerbation of symptoms, and (3) confirmed by a physician based on a structured physical exam.