EL PASO — Stanford University researchers say they've found the cause of a heart condition that's killed many young, healthy athletes. It's called hypertrophic cardiomyopathy and it's caused by a genetic mutation. They weren't sure how the heart muscle failed. When they harvested stem cells from patients with the mutation, the cells began to display abnormal rhythms and elevated calcium levels, suggesting a problem with calcium processing. Researchers say medications can help restore normal calcium processing, and that earlier treatment in those at risk for the condition may be better than waiting for symptoms to begin.