We all know that exercise is perhaps the single most beneficial action we can take to protect our health. So why are two of every three American adults still "sedentary"-meaning they get little or no exercise?

Live three years longer!

Most people who want to start exercising do so because it's "good" for them. But to stay motivated, you should know exactly why you want to start exercising.

For example, compared with people who exercise regularly, sedentary people are three times more likely to develop metabolic syndrome-a constellation of risk factors including high blood pressure (hypertension), elevated "bad" cholesterol, high blood sugar and obesity. Regular physical activity also has been found to reduce risk for cognitive decline.

And if that doesn't keep you motivated, consider this: People who regularly exercise briskly live an average of three years longer than those who are sedentary. "Briskly" means exercising at an intensity that makes you perspire and breathe a little heavily while still being able to carry on a conversation. This is known as the "talk test."

How much exercise?

It's a common misconception that you must exercise daily to achieve significant health benefits.

In a study of 10,000 men and 3,000 women conducted at the Cooper Aerobics Center's clinic, we found that walking just two miles in less than 30 minutes three days a week is all that's needed to achieve a "moderate" level of fitness, which lowers risk for all causes of death and disease.

For a less demanding workout that confers the same benefits, you could walk two miles in 35 minutes four days a week... or walk two miles in 40 minutes five days a week. If you prefer other forms of exercise, such as biking or swimming, use these frequency guidelines, plus the talk test (described above) to achieve a moderate fitness level. By increasing the frequency and/or intensity, you'll achieve even greater health benefits.

Hit the six-week mark

If you have not exercised regularly in the last six months and/or are overweight (for women, having a waist size of 35 inches or more... for men, 40 inches or more), the basic exercise requirement described above may be too much. You may want to start by walking only to the end of the block for a few days, then gradually increase the distance. Aim for an increase of up to 10% weekly-for example, from 10 minutes per week to 11 minutes the next week and so on.

Helpful: Expect the first few weeks to be miserable-you'll feel some muscle soreness for a while. Accept it-but make the commitment to keep going.

Important: If your muscle pain doesn't go away within several weeks, see your doctor to rule out an underlying condition, such as arthritis.

We've found at the Cooper Aerobics Center that few people quit after they've performed a program of physical activity for six weeks. Once people reach the four- to six-month mark, adherence to an exercise program approaches 100% for the long term.

Determine your baseline

If you've been sedentary, be sure to get a comprehensive medical checkup before starting an exercise program. This is particularly · important for men age 40 and older and women age 50 and older-cardiovascular disease risk rises at these ages.

People of any age with underlying health problems or a family history of diabetes, hypertension, high cholesterol or heart disease also should get a checkup before starting to exercise.

Ask your doctor-or a fitness trainer-to give you baseline measurements for strength, flexibility and aerobic capacity, which will enable you to track future changes.

Checking these measurements (along with such markers as blood pressure, cholesterol and blood sugar) again in about three months will give you tangible evidence of your progress and can motivate you to keep exercising.

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