Athlete's heart syndrome is enlargement of the heart muscle that develops in people who exercise regularly and vigorously. The changes in the heart may be mistaken for hypertrophic cardiomyopathy, which can cause life-threatening heart rhythms and sudden death. But athlete's heart syndrome is harmless.
When an athlete stops training, the heart returns to a normal size. In people with hypertrophic cardiomyopathy, the heart remains enlarged.