Mechanisms that typically result in increased cardiac output, such as inotropic support, increased heart rate, and decreased afterload, have a blunted effect in Fontan circulation. The "thoracic pump" is a contributor to venous return that has been largely unexplored in patients with Fontan physiology. Inspiratory muscle training can improve the performance of competitive athletes across a range of sports and can improve quality of life and functional capacity in heart failure patients, presumably by reducing inspiratory muscle fatigue and possibly by improving peripheral blood flow during exercise. One could surmise that the effects of these changes would be particularly important in the Fontan population. The investigators propose to study the effects of inspiratory muscle training on exercise and pulmonary function parameters in a cohort of adult Fontan patients.