Many people over the age of 50 have been watching the Olympics from their couches, resigned to be voyeurs, thinking they are too old to exercise themselves. But a study from Stanford University published in the Archives of Internal Medicinesuggests we may all be much better offwatching the Olympics from our treadmills. This study adds to the growing body of evidence showing that moderate exercise prolongs life, and decreases your risk of many diseases.

This study is unique because it looks at the positive effects of exercise on the elderly, at a time of life when many have already given up, believing (wrongly), that the damage has been done. Five hundred runners were tracked for more than 20 years. They averaged four hours of running per week at the beginning, just more than an hour per week by the end of 20 years. It turned out the elderly runners were half as likely to die prematurely from heart attacks, cancer, neurological disease including strokes, infections, and other causes than the non-runners.

Perhaps even more interestingly, there was no evidence that the patients in the study suffered from more arthritis or needed more knee replacements than non-runners. This is an important finding for aging people who insist they shouldn't exercise because it will wear down their joints.

The study is limited because it is observational and does not prove that it is actually the running that kept the patients alive longer. Other lifestyle factors may have contributed to longevity. People who run also tend to have better diets, better mood, better sleep habits, all factors that can decrease stress and influence lifespan, especially when combined with exercise.

But there is little doubt that exercise is good for you. It lowers stress, improves circulation, and decreases pressure on the heart. Some common questions my elderly patients ask me:

Q- If I start running tomorrow, will it help? A - It is never too late to start running. It won't help much right away, but the benefits accrue over several weeks.

Q- What about my knees and my back? If I'm overweight, won't it hurt them? A- You should check with your physician first. If he or she clears you, you should start slow and build up. I advise running on soft surfaces. You can be monitored for signs of arthritis, and the Stanford study suggests no increase over non-runners.

Q- Do I have to run? Can't I do something else that doesn't hurt as much? A- Swimming, elliptical, Nordic track, rowing machines, and bicycles are all good cardiovascular exercise which decrease pressure on the heart while improving circulation to the vital organs of the body. Each can be just as effective exercise as running and may be better tolerated in certain cases.

Marc Siegel MD is an internist and associate professor of medicine at the NYU School of Medicine. He is a Fox News Medical Contributor and writes a health column for LA Times, where he examines TV and movies for medical accuracy. Dr. Siegel is the author of False Alarm: the Truth About the Epidemic of Fear (Wiley 2005) and Bird Flu: Everything You Need to Know About the Next Pandemic (Wiley 2006). Read more at www.doctorsiegel.com