To the Editor: Drs Shephard and Balady1 discuss the potential dangers of excessive exercise and the appropriate dose of physical activity. They cite 2 reports showing that in healthy persons, prolonged exercise can cause myocardial fatigue with a temporary depression of myocardial function.2 3 In addition, recent research demonstrates that prolonged aerobic exercise may cause subclinical myocardial necrosis in individuals with no risk factors for cardiovascular disease.4 5 Evidence also exists that apparently healthy individuals who are not active enough to meet a traditional exercise prescription (structured vigorous activity) are at a high risk for subclinical myocardial damage caused by prolonged strenuous exercise.6 These findings may be of some importance: a recent well-conducted, randomized trial demonstrated that a lifestyle approach to …