Objectives: Chronic physical inactivity contributes to the deaths of nearly 1 in 10 Americans. In seniors, the single most common life-threatening disease is congestive heart failure and for these patients, abnormalities of diastolic function play a critical role in the pathophysiology of their disease. The primary investigator's previous research has demonstrated that: a) healthy but sedentary aging leads to atrophy and stiffening of the heart with reduced myocardial and chamber compliance; b) in contrast, highly competitive senior athletes had cardiac compliance that was indistinguishable from healthy young individuals suggesting that lifelong exercise training prevented this stiffening; c) even prolonged and intense exercise training started after age 65 failed to reverse age-related cardiac and vascular stiffening; d) cardiac stiffening begins in middle age (45-64) and can be substantially prevented by training 4-5 days/wk. The primary objective of this project, is therefore to identify sedentary individuals aged 45-64, and initiate an exercise program carefully designed to maximize effects on cardiovascular compliance and function. After this aim is accomplished, we will have established a novel, practical exercise training strategy designed to prevent the cardiovascular stiffening with aging, improve the functional capacity in our aging population, and ultimately to prevent Heart failure with preserved Ejection Fraction. Such a determination would have enormous public health significance since this condition is quite difficult to treat once established.