DEAR DR. DONOHUE: I read the letter from the 65-year-old who complained his muscles had become mush. It is true that many older people shrink and lose muscle mass. However, that doesn’t apply to all of us. I am 70, and I work out only by dynamic tension principles and have done so for 56 years. No weights. I am blessed with discipline, good hair and good skin. I walk an hour every day. My doctor has never seen a patient like me. I look and act like I’m in my 50s. — S.S.

ANSWER: Aging doesn’t have to doom people to loss of muscles and strength. People have to work at it to keep themselves strong. Many problems attributed to old age are problems that come from loss of strength — the inability to rise from a chair and the loss of balance with frequent falls, for example.

You mention your program is dynamic tension. Do you know that is a registered trademark of Charles Atlas Ltd.? I bet you followed the Charles Atlas exercises since you were 9. The story goes that Charles, in trying to devise a strengthening program for himself, considered how strong wild animals were without ever lifting weights. He devised his dynamic tension program that pits one set of muscles against another. His method worked wonders for him and apparently for you.

Another way to keep muscles strong is isometric exercise. In this kind of program, muscles work to their maximum capacity, but no movement is involved, no barbells or dumbbells are lifted. With isometrics, muscles don’t shorten or lengthen. An example is trying to move a desk that is firmly fixed in place. Pushing against the desk involves muscle work, but the desk doesn’t move an inch. Studies have shown that performing an isometric exercise at maximum exertion for one second strengthens muscles and makes them grow. A variation is to push against an immovable object for six seconds at two-thirds maximum exertion. The exercises are repeated five times during the day. Leg muscles exercise isometrically by sitting in a chair and pushing against the floor.

The downside to isometrics is they do not enhance athletic performance.

DEAR DR. DONOHUE: I am in my late 60s and recently have seen my doctor for my semiannual checkup. I appear to be in good health.

Through the years, I have experienced increased heart rate and shortness of breath with extra exertion. Recently I had a disturbing experience. As I struggled to climb a very steep hill, I felt like my chest was going to explode. After a few minutes of rest, I felt fine. Can you explain this?

ANSWER: I can’t dismiss chest pain like the kind you describe as a one-time, fluke event. You have to make sure your heart is healthy, even though you just had a physical exam. Call your doctor, and describe what happened. I’d knock off exercise until you’re seen.

DEAR DR. DONOHUE: I am now in my 89th year and still working out at the YMCA six times a week. Monday, Wednesday and Friday are for half an hour on a bike at a speed of 75 revolutions per minute. Then I do 500 sit-ups and crunches on five different pieces of equipment.

On Tuesdays, Thursdays and Saturdays I do a total of 750 repetitions on 25 different pieces of equipment.

I fight to keep my weight at 170 pounds. I am 6 feet tall. I feel great. My doctor said, “Keep doing what you’re doing.” Do you suggest any changes? — R.V.

ANSWER: Changes? No. I couldn’t imitate you if my life depended on it.

I have been getting many letters from older people who exercise at a level that would challenge Hercules. I can’t imagine being able to do what you do at 89 years of age.

Readers shouldn’t try to match this sort of program without permission of their doctors. I offer it as an example of what is possible for some elderly people.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com.