Most children with concussion recover spontaneously within 2-4 weeks of injury but approximately 30% experience prolonged post-concussive symptoms (PPCS), impacting quality of life. There are no evidence-based therapies for acute pediatric concussion. Instead, current consensus-based-recommendations include physical and cognitive rest as the mainstays of treatment despite evidence to the contrary that excessive rest prolongs recovery and that early (first week) activity reduces PPCS incidence. The investigators were the first to demonstrate that a systematic evaluation of exercise tolerance using the Buffalo Concussion Treadmill Test (BCTT) to prescribe a guided exercise prescription in those with PPCS safely sped recovery. The investigators have preliminary data that exercise may speed recovery if administered within the first week after sport-related concussion (SRC). Concussion is associated with adverse effects on the autonomic nervous system (ANS) and control of cerebral blood flow (CBF). The investigators have shown that sub-threshold aerobic exercise reduces symptoms in PPCS in association with normalization of ANS function and CBF control, providing a potential mechanism of action as well as target engagement for this non-pharmacological therapy. An important translational aspect is that the BCTT would be for clinicians a clinical proxy of concussion physiology that would help make the difficult return-to-activity/play decision more objective and physiologically-based. The investigators' long-term goals are to develop therapies that safely speed recovery from concussion, return youth back to their typical activities, actively prevent PPCS, and develop practical physiological tools for clinicians. The overall objective of this application is to determine if early guided aerobic exercise speeds recovery from SRC. The investigators specifically test underlying mechanisms of action for faster recovery by measuring ANS function (the cold pressor test, CPT) and CBF (by Arterial Spin Labeling, ASL). Age- and sex-matched healthy controls will be recruited to quantify physiological CBF differences and serve as a normative control group. The investigators will examine these questions in a randomized, multisite controlled trial in adolescents aged 13-18y with acute SRC by pursuing the following two specific aims: (1) determine if guided aerobic exercise/education speeds recovery after acute concussion in adolescents compared with (a) rest/education with a gradual return to activities, (i.e., current recommendation) and (b) a progressive stretching program/education (attention placebo); and (2) as secondary aims the investigators will quantify the effects of each intervention in Aim 1 on CBF and on autonomic function (CPT), which serve as relevant and objective physiological measures of autonomic regulation of CBF following concussion. This project will provide critical evidence to support the role of guided aerobic exercise for the active treatment of acute concussion and for a physiological mechanism(s) for its effect, which would lead to a paradigm shift from the current passive approach to treatment. The translational aspect of the project is unique and would have immediate impact to help clinicians determine recovery based on physiology rather than symptoms. A controlled active approach to concussion treatment may also substantially reduce the incidence of PPCS in children.