People exercise for many reasons: work, health, recreation, competition, or their appearance. The amount of exercise you need depends on your reasons for doing it, your starting point, and how quickly you want to achieve your goals, reports the December 2009 issue of Harvard Men’s Health Watch.

For work: We’ve replaced hoes with tractors, brooms with vacuums, and stairs with escalators. In the process, we’ve created a shortage of the physical activity that we need to ward off disease. Any physical activity you can get while doing work is good.

For health: Regular exercise provides protection against heart attack, stroke, high blood pressure, diabetes, and many other ills. The key is isotonic exercise—activities that use your large muscle groups in a rhythmic, repetitive fashion without making your muscles work against heavy resistance. Isotonic exercises include walking, bicycling, rowing, and the like. You can get all the health benefits you need from moderate exercise, even if you do it in little chunks of time. A range of activities can be counted toward daily exercise, from climbing the stairs to using an elliptical machine.

The key is to do enough, often enough.

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.

For competition: 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.

For appearance: You can lose weight with the moderate exercise you need for health—but for faster, more impressive weight loss, do about an hour of moderate exercise a day.
Harvard Men’s Health Watch notes that the amount of exercise youneed is the amount that allows you to meet your goals.

Read the full-length article: "How much exercise do you need?"