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"Lucy" is a fossilized female australopithecus, a member of an advanced genus of hominids who walked upright approximately 4 million years ago.1 Today, walking remains a major form of physical activity for women. We have witnessed, however, a recent decline in the use of bipedal motion � to the point where the Segway Human Transporter, a self-balancing personal-transportation device, is being evaluated by the U.S. Postal Service as a means for permitting letter carriers to make their appointed rounds without walking.2 Two articles in this issue of the Journal demonstrate that such developments are likely to accelerate the decline in physical activity in adolescent girls (as documented by Kimm et al.3) and to increase the incidence of atherosclerotic cardiovascular disease among middle-aged and older women (as shown by Manson et al.4). Manson and colleagues used questionnaires to measure recreational physical-activity levels in 73,743 women in the Women's Health Initiative Observational Study.4 Women were excluded if they had coronary heart disease or other selected medical conditions or were unable to walk at least one block. Participants were followed for an average of 3.2 years, during which time they had 1551 first cardiovascular events. Subjects were divided into quintiles of physical activity according to their weekly energy expenditure in metabolic-equivalent (MET)�hours. One MET equals a level of oxygen consumption of 3.5 ml per kilogram of body weight per minute and approximates the average energy expenditure during rest. The energy expended in walking at 3 mi (5 km) per hour, for example, is equivalent to 3.3 MET, or 3.3 times the resting energy expenditure. Women in the highest quintile of physical activity exercise for at least 23.4 MET-hours per week, which is roughly equivalent to walking at 3 mi per hour for an hour each day. As compared with the least active subjects and after adjustment for possible confounding factors, women in progressively more active quintiles had 11, 19, 22, and 28 percent fewer total cardiovascular events. There was a similar progressive reduction in cardiovascular risk with either walking or vigorous activity alone, although the distribution of walkers and vigorous exercisers precluded analysis by quintiles. According to the unadjusted data for total exercise, as compared with the least active women, women who spent between 45 minutes and 7 hours each week walking had 3.6 to 7.8 fewer cardiovascular events for every 1000 participants over a period of 3.2 years. This approximates the reduction in cardiovascular events by 4 for every 1000 women that was observed when healthy women were treated with lipid-lowering agents over a five-year period,5 although the analysis of exercise is not based on a clinical trial and ignores the fact that women in the Women's Health Initiative Observational Study probably participated in physical activity for longer than the observational period of the study. These results are important because of the size of the study population and the extension to black women of observations that have been made previously in white women. Black women composed 9 percent of the population � a sample size sufficient to document a decreasing risk of coronary heart disease with increasing exercise in these women. The authors caution that the results are based on questionnaires and that these volunteers may not represent a cross-section of women in the United States. In addition, all epidemiologic examinations of occupational and recreational physical activity are plagued by the possibility that people who elect vigorous lifestyles are different physiologically, emotionally, or genetically from their less active peers. Selection according to such a "hardiness factor" can be addressed only by a clinical trial � something that is unlikely to be undertaken, given the number of subjects required, the cost, and the absence of an obvious sponsor. There have been randomized, controlled trials of exercise in patients with coronary heart disease, however, and a meta-analysis of 51 trials of exercise-based cardiac rehabilitation documented a 27 percent reduction in total mortality and a 31 percent reduction in mortality from coronary heart disease over a mean follow-up period of 2.4 years.6 Such results, coupled with the extensive epidemiologic evidence and known biologic effects of physical activity, provide strong support for the benefits of exercise. But how much and what type is enough, and what specific recommendations should physicians make? The amount, intensity, and duration of physical activity required to reduce the risk of coronary heart disease are an area of intense debate largely because the available results are inconsistent. This lack of consistency is probably due to differences among the populations examined and the activity level of the "inactive" group, the range of activity among participants, subjects' ability to report episodes of vigorous activity more accurately than they can report milder forms of activity, and the use of questionnaires to classify the intensity of exercise.7 The Harvard Alumni Study, for example, demonstrated no further reduction in events associated with coronary heart disease in men with an energy expenditure of more than 2000 kcal per week8 � a value corresponding to the highest quintile of energy expenditure in the study by Manson et al. These two studies, one in men and one in women, show either a threshold effect8 or a progressive decline with progressive activity,4 possibly because of differences in the range of activity in the populations. Approximately half the studies that have been published suggest that vigorous activity is required for a reduction in the risk of coronary heart disease.7 In populations in which the reference group is relatively active, vigorous activity may be required in order to show a difference in coronary heart disease events,7 although Manson et al. found a benefit with both vigorous activity and walking. The classification of activity is also problematic. Many studies define vigorous activity as that requiring an energy expenditure of more than 6 MET, but less exertion can be vigorous for elderly and unfit persons. The study by Manson et al. defined vigorous activity as that producing sweating or tachycardia, but even these physiological responses depend on the level of fitness. Despite such methodologic problems, the overwhelming evidence supporting the beneficial effects of physical activity on the risk of coronary heart disease warrants the recommendation by health care professionals of regular exercise for most patients. The Centers for Disease Control and Prevention (CDC) and the American College of Sports Medicine (ACSM) recommend at least 30 minutes of moderately intense physical activity on most, and preferably all, days of the week.9 Moderately intense exercise is that which results in mild shortness of breath and involves oxygen uptake that is approximately 50 percent of the maximal level; an example of such exercise is brisk walking. This recommendation does not prohibit getting more exercise and was never intended either as a definition of the optimal exercise level or to discourage people from exercising more. The data presented by Manson et al. suggest that an hour per day of moderately intense exercise is closer to optimal, and most evidence suggests that progressive amounts of exercise lead to a progressive reduction in cardiac risk. A meta-analysis of 30 cohort studies involving more than 2 million person-years of observation demonstrates a nearly linear decline in the risk of coronary heart disease with increasing levels of physical activity.10 The National Runners' Health Study demonstrates progressive increases in high-density lipoprotein cholesterol and reductions in blood pressure and obesity in both men11 and women12 with increasing exercise levels that are considerably higher than those recommended by the CDC and ACSM. Consequently, although the CDC�ACSM exercise recommendations are prudent, physicians should, in my opinion, recommend them as a minimal level of daily exertion. A lower level of exertion is also beneficial,4 and more vigorous exercise, for those who are so inclined, is probably more beneficial, but also carries a cardiovascular risk, especially for those who are usually inactive.13 People should try to exercise daily to minimize the number of days missed and because many of the effects of physical activity on risk factors such as glucose levels, triglyceride levels, and blood pressure are, in part, acute effects of recent exercise.14 We, as a society, continue to evolve. Data such as those provided in this issue of the Journal suggest that the evolution of healthy lifestyles should include a hefty dose of one of our earliest evolutionary steps � walking and other forms of physical activity.
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