DEAR DR. DONOHUE: I am an active, 32-year-old male who has exercised since age 16. I lift weights, run and play a variety of sports. I fractured my left ankle playing baseball, and the doctor says I will be out of commission for four months. Already I see my muscles shrinking. How out of condition will I be in four months, and how can I minimize my losses? – E.D.

ANSWER: Anyone who has had a cast knows how rapidly unused muscles shrink. The cast loosens in a matter of weeks.

When people are forced to stop exercising, there’s a rapid decline in their muscle endurance within three weeks. The decline slows down somewhat after that, but it continues. With three months of inaction, people lose about half their muscle endurance.

The same applies to muscle size and strength. After three months of enforced rest, muscles lose about 50 percent of their strength.

You can salvage some of your body endurance by continuing with upper-body exercise. Uninterrupted muscle movements of the arms and shoulders for 15 to 30 minutes can maintain the heart’s pumping action close to the level it attained from your running program.

Weightlifting for the arms, shoulders, back and abdomen, if OK’d by your doctor, will arrest atrophy of those muscles.

As for the injured leg and its muscles, isometric muscle contractions can slow the decline in their strength and size. An isometric contraction is a tensing of the muscles without any movement. If the doctor approves of isometrics for your injured leg, then begin a program of isometric leg exercises.

Things are not so bleak. Once you can restart exercising, you can get back to your former status much sooner than can a person who has never trained. I can’t give you the time in weeks or months, but it won’t take as long to reach your pre-injury level as it took you to get there in the first place.

DEAR DR. DONOHUE: I have osteoarthritis. I am getting conflicting advice about exercise for someone like me. Some tell me exercise will make my joints worse. What do you think about this? – S.D.

ANSWER: Osteoarthritis is the most common kind of arthritis, the kind that comes from a disintegration of the cartilage cushions inside joints. It’s often called wear-and-tear arthritis. That implies that joint cartilage degenerates from the daily use of the joints. That’s not the true picture.

Exercise and joint use stimulate cartilage repair. An inactive life is not good for anyone, and that includes someone with osteoarthritis.

Furthermore, exercise strengthens muscles, and strong muscles protect joints and keep them mobile. If people with osteoarthritis stay sedentary, they are twice as likely to lose their ability to perform daily activities, like walking, as are those who exercise regularly.

Exercise has to be tempered with common sense. If exercise brings on pain, don’t do it. The “no pain, no gain” maxim is foolishness.

Low-impact exercise is the kind of exercise for people with osteoarthritis. “Low-impact” means you should not be jumping up and down on a hard surface. Walking, biking and swimming are low-impact exercises and the kind suitable for someone like you.

Even weightlifting is encouraged so long as it doesn’t cause joint pain.

DEAR DR. DONOHUE: My grandson gets stretch marks on his upper arms from lifting weights. Is there anything he can do to prevent them? Can he make them go away? – P.S.

ANSWER: Stretch marks are skin scars that come from enlarging muscles’ splitting the skin slightly. They are actually evidence that a weightlifter is attaining the goal of increasing muscle size.

I don’t know how to prevent them. I have seen that the acne medicine Retin-A might make them shrink. I can’t guarantee that. They become less noticeable in time. They lose their red color.

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.