This is a multi-center prospective cohort study of patients with first-episode deep venous thrombosis and pulmonary embolism.

Subjects will be identified from the clinical setting and approached to participate in this observational study where participants will be enrolled at 3 different sites and referred from several more sites and have: cardiopulmonary exercise testing and pulmonary function testing at The Institute of Exercise and Environmental Medicine (IEEM), UTSW Exercise Facility, Cardiac MRI and MRI for pulmonary perfusion at Children's Medical Center and MR Spectroscopy and MR for Muscle Perfusion at the Advanced Imaging Research Center (AIRC) performed in Dallas over a 3 day research visit at week 12 and Month 12. Blood is collected for biomarkers at these visits and multiple questionnaires are completed by participants.

Inclusion Criteria:

• Ages 8 to ≤ 21 years
• Participant must be able to speak and understand English
• Be willing to participate and able to comply with the study protocol
• For participants with PE: Children with acute, radiologically confirmed pulmonary embolism (PE) with our without DVT
• For control group: Cohort 1: Children who are prescribed physical activity restrictions for 2 up to 12 weeks following any minor outpatient surgery or, minor injury (surgery or injury is referred to as "diagnosis" hereafter) Cohort 2: Children who are not prescribed physical activity restrictions and are otherwise considered to be healthy.

Exclusion Criteria:

• Congenital heart disease with abnormal pulmonary circulation or with in-situ pulmonary artery thrombosis
• Chronic kidney disease
• Chronic inflammatory or an autoimmune disorder (such as systemic lupus erythematosus, juvenile rheumatoid disorder, inflammatory bowel disease, and sickle cell disease)
• A metabolic or endocrinological disorder such as diabetes mellitus or thyroid disorder
• History of or active cancer
• Pregnant
• Musculoskeletal limitations to exercise expected to be present uptil 4 months post-diagnosis
• Weight ≥ 300 lbs
• Contraindications to magnetic resonance imaging
• Frequent severe exacerbations of asthma defined by two or more bursts of systemic glucocorticoids (more than three days each) in the previous year or at least one hospitalization, intensive care unit stay or mechanical ventilation in the previous year. Patients should also be excluded if there are daily symptoms of asthma requiring daily use of short-acting bronchodilators such as albuterol or levalbuterol administration. The use of controller medications such as daily inhaled corticosteroids for mild persistent asthma is not exclusionary.
• Has any other medical condition, which in the opinion of the investigator may potentially compromise the safety or compliance of the patient or may preclude the patient's successful completion of the clinical study

Additional exclusion criteria for participants with PE:

• Prior history of DVT or PE (upper extremity, cerebral sinus venous thrombosis and abdominal thromboses encountered as a neonate are not exclusion criteria)
• Lack of anticoagulant treatment for the acute VTE due to contraindications

Study population will include children (defined as persons who have not attained the legal age for consent to treatments or procedures involved in the research, under the applicable law of the jurisdiction in which the research will be conducted), and patients (defined as individuals in a clinical setting with whom there is a treatment relationship).

Participants must meet the eligibility criteria in order to participate in this trial.