DEAR DR. DONOHUE: Please solve vastly different opinions on water versus land aerobics. I used to walk 5 miles a day and ended up with blisters and chaffed parts of the body. Is water aerobics as beneficial as land? – V.M.

Water aerobics is an excellent aerobic exercise, the kind that gives the heart a workout, burns calories and lowers blood pressure. It has the added advantages of putting less stress on joints and not being a threat of serious injury if a person falls.

Brisk water aerobic exercise can burn 300 to 500 calories in an hour. That’s a good number of calories, and it compares equally to land aerobic exercise.

People can exercise in deep water by wearing a flotation belt. The belt keeps people vertical. Exercising in chest-deep (or waist-deep) water still offers the same benefits as deep water, and the calorie cost is more because the body is not completely buoyed by water.

The heart rate in water exercise is 10 beats less than it would be in land exercises. In water, more blood shifts to vessels in the chest, and more blood is siphoned into the heart. One beat of the heart, therefore, pumps more blood than it would if the exerciser were on land. If you take your pulse to measure how much training your heart is getting, remember to subtract 10 beats from the heart rate goal of land exercise.

Water exercise is perfect for people who are recuperating from a leg injury. They can maintain their conditioning while they are recovering.

You don’t have to abandon all land exercise. It does have one advantage. It is a better way to prevent osteoporosis, since the body’s bones must support body weight without the buoyant effect that water gives.

DEAR DR. DONOHUE: I have just started a weightlifting program, and my friends tell me I should wear a back belt – one of those wide leather belts many lifters wear. They tell me it will prevent back injuries. Is there any proof of that? – E.K.

A weightlifting belt is said to prevent low back injuries and back muscle strains. It is also supposed to assist a lifter in lifting heavier loads. There is not a whole lot of support for either statement.

The belt increases pressure within the abdomen, and that, in turn, decreases the stress that the back discs take while lifting. The discs are doughnutlike shock absorbers that lie between adjacent backbones.

Such belts ought to be worn only when lifting very heavy loads or when performing a series of lifts to the point of muscle exhaustion. Wearing one at all times cancels the stimulus for strengthening abdominal and back muscles.

DEAR DR. DONOHUE: I am 16, and I want to start lifting weights. My mom does not want me to do so. She’s afraid I might hurt myself. She said to write to you, and she would accept your opinion. So I am writing. And I would appreciate any tips you can give me. – D.T.

It’s safe for a 16-year-old to lift weights if he has someone who can instruct him how to lift correctly and sensibly.

As for tips, I can give you a few. A coach would be a better source of information.

Begin by determining how much weight you can comfortably lift once. Then take 60 percent of that weight and use it for eight consecutive lifts. Take a break, and repeat the eight lifts twice more with a break again between the second and third sets of lifts.

How much of a break? For light weights, a minute or less of rest is all that is necessary. For very heavy weights, take a four-minute break. For moderate amounts of weight, a two-minute break suffices.

Vary the kinds of lifts you do. You’ll find many books in your local library that describe them for you. Don’t exercise the same muscles two days in a row. You have to give muscles a chance to grow, and they do so when they are resting.

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.

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