You are established, comfortable in your career or retirement... and sedentary.
Too busy to fit regular exercise into your schedule, you make up for it with a vigorous workout once in awhile. It is better than no exercise at all, right?

Probably not. Exercise clearly lowers your risk of heart disease if you do it regularly and in moderation. But new evidence suggests that if you are out of shape, occasional strenuous physical exertion, such as shoveling snow or running, is risky.

Risks stem from overdoing it

New data from Harvard study show heart attack risk among sedentary people was more than 100 times greater during strenuous exertion than during light or no exercise. Researchers theorize that heavy exercise may trigger disruption of plaque, causing formation of a blood clot that can block coronary arteries. The risk seems to be greater in people not used to vigorous exercise.

Being in shape, however, may lower your risk. The study found increasing the frequency of activity led to, a progressively lower risk of heart attacks.

In people who regularly exercise at least five times a week, heart attack risk was only 2.4 times greater during strenuous exertion than during light or no exercise.

But here is an important perspective: Most heart attacks do not occur during exercise. Of all heart attacks, 96 percent occur at rest. The minority of heart attacks - only 4 percent - occur during strenuous exertion. Most people who have heart attacks during activity are sedentary or have underlying heart disease and overdo it.

What it means to be "fit"

Fitness means a well-conditioned cardiovascular and muscular system. Both your heart and muscles need regular stimulation to stay fit. You are fit if you can:

• Carry out daily tasks without fatigue and have ample energy to enjoy leisure time pursuits.
• Walk a couple of blocks or climb one or two flights of stairs without becoming "winded" or feeling heaviness or fatigue in your legs.
• Carry on a conversation during light to moderate exercise such as brisk walking.

If you sit most of the day, you are probably not fit. Signs of deconditioning include feeling tired most of the time, being unable to keep up with others your age, avoiding physical activity because you know you will quickly tire, and becoming short of breath or fatigued with walking a short distance.

Remember the terrible "toos"

The risks of exercise stem from doing too much, too vigorously with too little gradual preparation.

If you have been inactive, the American College of Sports Medicine recommends a stress test if you are a men older than 40 or a woman older than 50, or you have two or more risk factors for heart disease such as smoking, high blood pressure, high cholesterol, excess weight, family history or underlying health condition.

A properly designed and well balanced physical conditioning program based on a correctly assessed physical fitness ability, could be of major value, increasing the effectiveness of your workout and decreasing the risk of injury.

But remember most heart attacks occur during rest. Instead of worrying about risks, use this latest information as an incentive for getting in shape with regular activity.