Inclusion Criteria:  generally healthy sedentary (defined as being involved in less than 30 minutes of moderately-intense physical activity per day) additionally for the participants with Down syndrome: diagnosis of Down syndrome trisomy 21 and normal thyroid function or stable thyroid function (and medications) for at least 6 mo.  Exclusion Criteria:  congenital heart disease; atherosclerotic or other vascular disease; asthma or other pulmonary disease; hypertension (defined BP >140/90 mmHg); blood pressure below 90/60 mmHg; history of pre-syncope or syncope; diabetes (defined as Hba1c of >7.5% or use of glucose lowering medication); severe obesity (defined as BMI >40); medications affecting heart rate, blood pressure or arterial function; anti-inflammatory medication including NSAIDS; current smoking and pregnancy.