This study will examine how heart stiffness and a weak atrium affect exercise capacity and symptoms in patients with hypertrophic cardiomyopathy (HCM). The atrium is the booster pumping chamber of the heart that helps the ventricle (main pumping chamber), to fill properly. HCM is an inherited disease in which the ventricle becomes thickened and, in some patients, stiff. The stiffness makes it difficult for the ventricle to fill and empty, causing breathing difficulty, fatigue, and reduced exercise capacity. Scar formation and a weakened atrium can cause the heart to stiffen. Information gained from this study may guide doctors in prescribing medicines to reduce scarring or improve atrial function.