Many studies have found regular aerobic exercise and fitness have many cardiovascular benefits in both men and women, and may reduce the risk of heart attacks or other coronary events. Yet, paradoxically, in addition to benefits, vigorous exertion may also trigger sudden death. Sudden death from cardiac causes often occurs during or just after physical exertion -- including vigorous exercise.

• These are usually caused by various (often) unsuspected structural heart diseases.

• However the absolute risk is rather low - 1 sudden death per 1.5 million episodes of vigorous activity.

• This short-term risk is lower in those who lead an active lifestyle characterized by habitual vigorous exercise compared to persons who are not accustomed to regular exercise.

Research based on the Physicians' Health Study found that regular physical exercise does not provide long-term protection against coronary events; however, this may be explained by certain design features of the study. For example, exercise levels were (self) reported by the test subjects; there were no objective measures of physical fitness; and physical activity levels could well have varied over time.

The benefits do not come without some risk, particularly when vigorous exertion is undertaken abruptly by untrained or previously sedentary persons.

Nevertheless, the balance of the evidence thus supports the value and importance of participation in regular exercise regimens.

Source: Barry J. Maron, "The Paradox of Exercise," New England Journal of Medicine, Nov. 9, 2000.

For text http://www.nejm.org/content/2000/0343/0019/1409.asp