Exercise: It's cheap, readily available, and the single most effective step nonsmokers can take to avoid chronic and potentially fatal diseases. If it were being hawked on late-night television, you'd think the phone lines would be tied up for hours.

But regular physical activity remains a hard sell. Despite mounting evidence that it lowers the risk for obesity, heart disease, diabetes, depression, and many forms of cancer, the average citizen is increasingly sedentary. Still, the Department of Health and Human Services (HHS) isn't giving up on us. In 2007, the agency convened an expert committee to evaluate a decade of scientific evidence on the benefits of physical activity. Committee member Dr. I-Min Lee, who also serves on the Harvard Women's Health Watch advisory board, said she and her colleagues found an "impressive range of health benefits coming from being physically active." They submitted their findings, and in the fall of 2008, the government issued a detailed exercise prescription for the nation.

The ground rules

The "2008 Physical Activity Guidelines for Americans" (www.health.gov/PAGuidelines) are more extensive than those of most other health organizations, and more extensive than earlier HHS recommendations. While assuring us that a couple of hours a week of moderate activity provides important health benefits, the guidelines also stress that more is indeed more—finding added benefit from exercising longer and doing so with greater intensity. As Dr. Lee puts it, "Any physical activity one can do is good, but more is better."

The guidelines are also more inclusive. They apply not just to the standard adult audience but to almost everyone age 6 and over—children, adolescents, pregnant women, seniors, and people with chronic diseases and disabilities. This time around, no one gets off the hook. Adults, whether 18 or 81, are urged to get no less than 150 minutes (two and a half hours) of moderate activity or 75 minutes (one hour and 15 minutes) of vigorous activity—or some combination of the two—each week (see chart). Sessions should last at least 10 minutes and be spread evenly through the week. Adults are advised to fit in two weekly sessions of strength training as well. The authors urge even people with medical conditions to meet these standards, though they acknowledge this may require a gradual buildup. (See "Exercising with health conditions.")

Children and adolescents should be getting even more than adults: at least an hour a day (420 minutes per week), including both aerobic activity and exercise that builds muscle and bone.

The guidelines also distinguish between different levels of physical activity. For example, for most middle-aged adults, moderate-intensity aerobic exercise is comparable to walking three to five miles an hour; vigorous exercise is anything higher. Or, on a zero-to-10 scale, with zero as the amount of activity involved in sitting, and 10 as the effort of running at top speed, moderate exercise begins at five and vigorous exercise at seven. Perhaps the simplest way to distinguish moderate from vigorous exercise is by trying to speak as you work out: if you can talk while working out but have a hard time singing, you're exercising moderately; if you find it difficult even to talk, that's vigorous.

Strength training should involve all the major muscle groups of the legs, hips, back, chest, stomach, shoulders, and arms. You should repeat each exercise for each muscle group eight to 12 times. Many aerobic activities also provide strength training: for example, race-walking strengthens legs and hips; rowing builds muscles in the arms, legs, chest, and shoulders.

How much is enough?

Go beyond the basics if you can, say the guidelines. Once you're routinely logging the recommended levels of aerobic activity, start to add a few minutes a day. (Ramping up slowly reduces the likelihood of injury.) The HHS committee found that you can get even greater health benefits and more effective weight control when you reach twice the recommended weekly amount—that is, 300 minutes of moderate activity, 150 minutes of vigorous activity, or a combination of the two. More than that may be even better.