DEAR DR. DONOHUE: I wonder if walking in a pool for more than an hour is equivalent to walking on pavement. I have taken up this activity because of the high humidity in my area of the country. I walk, jog and do exercises in the pool. I can’t find any information on comparing land and water exercise. – E.C.

ANSWER: My immediate reaction to your question is to say they are not the same. On dry land, you carry your body weight forward with each step. In water, buoyancy props up body weight, so moving forward requires less effort. In water at neck level, body weight decreases by 90 percent; at chest level, by 65 percent; at waist level, by 50 percent.

Looking into this in the literature, however, makes me modify my position. Moving in water requires a person to move against greater resistance than does land movement. On land, a person moves only against air.

In addition, to maintain an upright position in water, your core muscles have to work hard and work constantly to keep you erect. The core muscles include the abdominal and lower back muscles.

Exercising in water doesn’t raise the heart rate as much as exercising on land. The reasons for that are many. But at an equivalent workout in water to that on land, the heart beats seven to 12 beats a minute slower.

I have to mention a great benefit of water exercise – joint protection. The impact of motion in water is much less than the impact of motion on land.

What’s the upshot of all this? You get a good workout by exercising in water. An hour of water walking, for instance, burns from 300 to 500 calories. Whether water exercise is the same as, better than or not as good as land exercise seems to me to depend on what authority you happen to read.

DEAR DR. DONOHUE: I am a 78-year-old male who’s 6 feet 1 inch tall and weighs 225 pounds (the same since my early 20s). I have been very active in almost all sports since I was a young boy. I have always worked out with weights, gone hiking and done bicycling to maintain my conditioning.

I have been fighting arthritis since my early 30s and now find myself unable to maintain much activity without serious problems. What can I do to stay in shape? – R.N.

ANSWER: I see from your address that you live in a warm climate throughout the year. Why not take a leaf from E.C.’s book? Exercise in water. You can even lift weights in water. There are weights specially designed for water exercise, not the same kind of weights you’d use in a gym.

Swimming is an excellent conditioner and not hard on joints. You’re never too old to learn how to swim.

DEAR DR. DONOHUE: My wife and I attended a presentation on knee replacement some time ago. At this presentation it was stated that walking, running and jogging can be harmful to knees and a preferred exercise is a stationary bicycle. Our question: Is this type of exercise a preferred form of lower-body exercise and perhaps a good cardiovascular exercise? We are both in our mid-70s and reasonably active. We would appreciate your comments on this. – B. and A.W.

ANSWER: A stationary bike is an excellent way to exercise both for leg strength and for heart health. If you have knee problems, try the bike first before you purchase it. Pedaling can cause knee pain. Certainly biking doesn’t stress the knees as much as pounding the pavement does in jogging or running, but you should make sure your knees are up to it before making any investment in a bike.

You shouldn’t do any exercise that hurts. You can do any exercise that doesn’t.

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.