DEAR DR. DONOHUE: I am an active, 42-year-old woman and used to be a long-distance runner. I had to give it up. It was too hard on my knees. I happen to live on a lake, and I have taken up rowing. I love it, but I wonder if the amount of exercise I get from rowing is comparable to what I got from running. Is it? – P.J.

ANSWER: What I have to say about rowing a boat applies to rowing machines as well.

You are engaging in one of the most demanding of all exercises. Rowing increases muscle strength as well as conditions the heart. Many state that it is the best all-around conditioning exercise there is.

One of the beauties of rowing is that it allows a person to control the intensity of exercise by increasing or decreasing the number of strokes rowed per minute. If a person is just starting out, 12 strokes a minute is a good limit. You don’t have to do 12. If all you can row is three, great. As you work at it, aim for a pace of 20 to 25 strokes a minute. The more rapid the strokes, the less time a person is able to keep at it, but doing many strokes per minute produces a good muscle workout and a good heart workout.

A 137-pound (62-kg) woman burns from seven to 10 calories a minute while rowing. Minute by minute, that’s the equivalent of the amount of calories she would expend running a marathon in three hours and 45 minutes. If a person can run that fast and that long, invitations to marathon events will flood his or her mail.

You are not working only the upper body. Your thigh muscles, buttock muscles, abdominal muscles and back muscles as well as your shoulders and arms get in on the act.

If you tried, you would be hard-pressed to find another exercise that provides such benefits. Furthermore, you are protecting your knees.

DEAR DR. DONOHUE: I am a football player, and I have seen many players get the wind knocked out of them. It happens more in practice than in actual games. They look like they can’t breathe. What exactly happens, and how do you go about helping someone who has had the wind knocked out? – R.D.

ANSWER: When a person inhales, the diaphragm muscle descends. The diaphragm is the horizontal sheet of muscles separating abdomen from chest. Its downward motion creates negative pressure within the chest and lungs. Air rushes in. That’s how we inhale.

A blow to the upper center of the abdomen shocks a network of nerves lying there. One function of that nerve network is control of diaphragm movement.

The blow also causes a spasm of the diaphragm. Interruption of nerve control together with muscle spasm makes it impossible to lower the diaphragm to draw in air. That person has had the wind knocked out.

When this happens, tell the person to relax and to slowly breathe in through the nose and out through the mouth. Getting such a person on his or her feet helps relieve the diaphragm spasm.

DEAR DR. DONOHUE: I have a question that’s been on my mind for weeks. I am a 12-year-old girl who weighs 118 pounds and is 5 feet 5 inches tall. Every day throughout the year I swim for a competitive swim team for about two to three hours. I think I am just a little fat for my age and need to lose a few pounds. My friends think I am perfect and would love to have my body. I really need a reply fast. – M.P.

ANSWER: Don’t lose an ounce. You are perfect.

I calculated your BMI – body mass index. It’s a way to assess how much of your weight is bone, muscle and body organs and how much is fat. You have a BMI of 19.6. That’s excellent. The normal BMI ranges from 19 to 25.

If you lose weight, you are going to lose muscle weight. Your swimming is going to suffer. What in the world makes you think you are too fat?

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.