DEAR DR. DONOHUE: I injured my knee about a week ago and am in a castlike contraption that makes it hard to exercise. I plan to play my senior and third year of varsity football in the fall. I’m going to be way behind my buddies who are exercising every day. What can I do to stay in shape? — M.R.

ANSWER: Immobilization leads to regression of physical fitness. But you’re not completely immobile.

First, I’ll give you the bad news. If you are among the elite fit, three months without exercise decreases your aerobic fitness by 50 percent. Aerobics is the kind of fitness that permits endurance. Long-distance running, long-distance swimming and any sport where the exerciser moves more or less constantly are considered aerobic exercise.

Strength is a different issue, but the facts are similar to those for aerobics. If you had performed strength training by lifting weights for six months and then stopped for three months, your strength would decrease by half of what it was at its peak.

I don’t think either of these applies to you. You plan to be out of your contraption by the start of school. That’s not three months. Furthermore, you can still be active. You can maintain aerobic conditioning by nonstop exercising of your arms. You use light weights and do exercises that increase your heart rate and breathing rate but don’t exhaust you. You can do arm-strengthening exercises, too. Those exercises transfer some benefits of strength training to the legs, believe it or not. That’s been demonstrated in people who had bad arthritis of the hips and knees, so strength training of their legs was not an option for them.

Strength comes back relatively quickly once you can start leg strength training. Muscles have memory. That memory permits restoration of muscle strength more quickly than it would be restored had you never exercised.

DEAR DR. DONOHUE: I read everywhere that protection of the skin from the sun is important. How about protection of the eyes? I mean sunglasses. What kinds are beneficial? Does the color of the lens make a difference? — R.O.

ANSWER: The eyes do need protection from the sun’s ultraviolet rays. Make sure the sunglasses filter all ultraviolet rays, both ultraviolet A and ultraviolet B.

As for lens color, amber, orange, gray, green and brown block out blue light, another kind of light that is said to be deleterious to the eyes. The lenses should be dark enough to make it impossible to see your eyes in the mirror, but not so dark that you can’t see stoplights and curbs clearly.

Wraparound glasses are best because they deflect light coming from the sides.

The shadow cast by a wide-brimmed hat cuts in half the number of UV rays reaching the eyes. So does a baseball cap if the brim faces forward and not backward, as seems to be the universal style.

DEAR DR. DONOHUE: I recently had arthroscopic surgery on my knee. I used to walk two to five miles daily, but my doctor wants me to either swim or bike instead. I have chosen a recumbent bike.

Mile for mile, how does biking compare with walking? — V.T.

ANSWER: The two are comparable. To give an accurate comparison, I’d have to know your weight, how fast you walk, how fast you pedal and how long you do both. Since you have to support your body weight in walking, it has a slightly higher calorie-burning effect. You can overcome that by pedaling faster.

Make your own judgment on the intensity of your exercise. If you can exercise to the point where it feels moderately hard but is not making you breathless, you’re getting enough exercise.

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.

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