How much exercise do you need?

Your brother plays tennis every weekend, and your son hits the gym three times a week. Your wife walks every day, and your daughter takes yoga and dance. You putter in the yard and enjoy a round of golf now and then, but you've finally decided to get serious about exercise.

To put good intentions into action, you need a goal. How much exercise do you need? Ask the U.S. Surgeon General, the Institute of Medicine, the American Heart Association, and the American College of Sports Medicine, and you're likely to get four different answers. But don't let that turn you off. In fact, you're the only one who can decide just how much exercise is best for you.

Why exercise?

People exercise for one of five reasons: for work, for health, for recreation, for competition, or for their appearance.

The amount of exercise you need depends on your reasons for exercising, on your starting point, and on how quickly you want to achieve your goals. And the type of exercise you choose depends on your personal abilities and preferences, on your schedule, and on the facilities at your disposal.

Exercise for work

For better or worse, not many 21st century Americans fill their exercise quotas in the workplace. As recently as the 1850s, about 30% of all the energy used for agriculture and manufacturing in the United States depended on human muscle power. No more. We've replaced hoes with tractors, brooms with vacuums, and stairs with escalators. Freed from physical work, people have used mental work to create a society of enormous convenience and comfort. In the process, though, we've created a hidden energy crisis — not a shortage of fossil fuels, but a shortage of the physical activity the human body needs to ward off disease and reach its full potential.

Exercise for health

Exercise is the best-kept secret in preventive medicine. Despite our other differences, we all need to exercise for health. Regular exercise provides essential protection against many of the diseases that plague our country. The list includes:

• heart attack

• stroke

• high blood pressure

• diabetes

• obesity

• osteoporosis and fractures

• depression

• colon and breast cancers

• dementia (memory loss).

What does it take to get these benefits? Less than you might think. The key is what exercise scientists call isotonic exercise — activities that use your large muscle groups in a rhythmic, repetitive fashion without making your muscles work against heavy resistance. We used to call this "aerobic" exercise because we thought it had to be intense enough to boost your heart rate into the aerobic range (70% to 85% of your maximum heart rate). We also called it "endurance" exercise because we thought it had to be sustained continuously to be beneficial. But we now know that neither of these long-held beliefs is true. In fact, you can get all the health benefits you need from moderate exercise that won't make you huff and puff, even if you do it in little chunks — as long as it adds up to enough total activity.

We coined the term "cardiometabolic exercise" (CME) to encompass a range of activities, from climbing the stairs in your office building to pushing yourself on an elliptical. All these things will improve your heart, your metabolism, and your health. The key is to do enough and to do it often enough. For health, doctors should "prescribe" at least 30 minutes of moderate exercise or 15 minutes of intense exercise a day. To see how your exercise stacks up, use the CME point system (see below), aiming to get at least 150 CME points a day.

Mix daily activities, formal workouts, and sports play to get the cardiometabolic exercise you need for health. And for best results, do some stretching nearly every day and some strength training two or three times a week. The older we get, the more we need these supplementary activities. And as the years roll on, most of us will also benefit from some simple exercises to improve balance and prevent falling, a major health problem for seniors.