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.

Exercise for recreation

No need for a point system, clock, or calendar here. If you're exercising for the fun of it, just go for it — as long as you meet your minimum needs for health.

But the recreational value of exercise brings up a point that's relevant for hard-working men who are "too busy to exercise." Exercise is a great way to dissipate stress and lift your spirits. If your work threatens to overload your psyche, consider using exercise to refresh your mind. For some, that will mean a trip to the gym to burn off some stress on a treadmill or elliptical; for others, it will be a walk or jog outdoors to get away from it all; and for others, it will be a bit of stretching or yoga at bedtime. But don't let exercise add to your stress; if you hit a truly overwhelming patch, you can take a few days off without losing your edge.

Baseball legend Yogi Berra got it right when he said exercise is 90 percent mental — and the other half is physical.

Exercise for competition

Here's where aerobic training comes in. To stay well, exercise for health fitness. To hit your peak for road running, racquet sports, basketball, biking, or any other competitive sport, work out for aerobic fitness. That means boosting your heart rate to 70% to 85% of its maximum and holding it there for 20 to 60 minutes. If you're like most of us, you'll have to build up slowly, and everyone who works out this hard should warm up before and cool down after aerobics. You'll also benefit from stretching, strength training, and if you're really going for it, interval training, or speed drills.

It's a lot to ask of your body, particularly as you get older. And strenuous exercise has potential pitfalls. Every man over 50 should get medical clearance before starting an intense exercise program. People with diabetes, high blood pressure, obesity, abnormal cholesterol levels, or other cardiovascular issues require special care. And because intense exercise is more likely to cause problems than moderate exercise, it's particularly important for competitive athletes to listen to their bodies and react promptly to signals of distress.

Aerobic exercise has done a lot of good for many people. But because it is demanding, it has discouraged many others from exercising at all. That's why everyone should exercise for health and fitness, but only the motivated (and healthy) among us can set high-level aerobic fitness as a realistic goal.

Exercise for appearance's sake

Weight loss is the most common goal. You can get there with the moderate exercise you need for health — but for faster, more impressive weight loss, double your goal to 300 CME points, or about an hour of moderate exercise a day. It sounds like a lot, but remember that you can break it into chunks. Remember, too, that the little things you can build into your daily routine will make a big difference; climbing stairs and walking for transportation are prime examples. And to really make progress, cut down on the calories you consume as well as boosting the calories you burn with exercise.

Sorry to say, you can't selectively shed fat from your belly, butt, or thighs. But you can use calisthenics and strength training to firm up your muscles, which will make you look thinner and better.

Exercising your options

So how much exercise do you need?

Just enough to meet your goals. Make health your priority, and remember to get a check-up before you start a big new exercise push. Choose the activities that best fit your schedule, your budget, your abilities, and your taste. Construct a balanced program by adding the weight training, stretching, and exercises for balance that you need. Start slowly, build up gradually, and — above all — stick with it. As Yogi might have said, exercise is 50% ability and 90% persistence.