When it comes to exercise, everybody's got an excuse. Or 10. "I had a list," says Farai Chideya, a 36-year-old Los Angeles journalist who recently began lifting weights and taking dance classes. "I'm too busy. Maybe I'll hurt myself. Shouldn't I be out meeting new people instead?"

Despite the well-documented health benefits of exercise, fewer than half of adults in the United States get the minimum amount necessary for those rewards: 30 minutes of aerobic activity, most days a week. A quarter of Americans are sedentary. And the older people get, the less likely they are to exercise. That's a lot of people with a lot of excuses. And yet few of them are valid, say experts. There are just not that many people who truly can't exercise. The next few pages offer a field guide to overcoming inertia. Be honest. Your favorite excuse is probably among them.

Why bother?

Imagine the line outside the office of a doctor who is dispensing a treatment that incontrovertibly shows it can help prevent chronic diseases and early death. That's what a comprehensive review of medical research published in March in the Canadian Medical Association Journal said about regular physical activity. The authors found that there was "irrefutable evidence" that consistent exercise lowers the risk of illnesses including heart disease, cancer, osteoporosis, depression, high blood pressure, diabetes, and obesity. The U.S. Centers for Disease Control and Prevention recommends moderate-intensity exercise, which means your heart rate gets going but not so much that you are huffing and puffing and unable to carry on a conversation. Think biking, inline skating, using an elliptical trainer, and swimming. Strength training like lifting weights can also help prevent bone and muscle loss and shore up joints stressed by arthritis or pain (box, Page 61).

While the link between better health and exercise is clear, scientists are still nailing down exactly how physical activity reduces the risk of certain diseases and which of its effects stem in part from the weight loss that can result when working out is coupled with good nutrition. Several different factors, for example, most likely reduce the odds of developing heart disease. Exercise increases blood flow, for one, which stimulates the release of a chemical that relaxes artery walls and lowers blood pressure. It also seems to egg on the release of an enzyme that improves cholesterol balance, driving more of the "good" kind and less of the artery-clogging "bad" kind. It may also cut systemwide inflammation, which has been implicated in heart disease and a host of other ailments. And, of course, exercise can help control weight, which reduces the harmful effects of excess fat.

With cancer, the mechanisms are less clear. Exercise may limit estrogen circulation, which can stimulate some types of breast cancer, possibly through reducing body fat. And it may lower colon cancer risk by keeping the digestive system active, cutting the exposure of colon tissue to cancer-causing agents in food. "There is so much evidence that allows us to really prescribe exercise for someone," says Paul Ribisl, chair of health and exercise science at Wake Forest University in Winston-Salem, N.C. "The most difficult issue challenging us is how to change [people's] physical activity and eating patterns."