Dear Dr. Donohue • My son, 13, is an avid tennis player. I was watching him play the other day and noticed he was moving awkwardly. Then I watched him when he was at home, and he walked kind of funny. I asked him about this, and he said his knee hurt. He didn't bang it or injure it. He said it started about a week ago. He doesn't want to see a doctor. Should he? What could this be? — M.M.

Answer • Yes, he should see a doctor. He might have a common injury of young athletes, osteochondritis dissecans.

The inner cartilage covering the knee joint and the bone beneath it separate from the main body of the bone, the tibia — the larger of the two leg bones. It can happen on either side of the knee. Pain on weight-bearing is the prominent symptom.

The cause is believed to be repetitive microtrauma to the knee, something that is quite possible to an avid tennis player. It happens to children between the ages of 9 and 18. It also can happen to adults, but not as commonly.

If this is the diagnosis, the boy must rest the knee. Usually, a knee brace is applied to protect the knee from further stress. The period of bracing lasts four to six weeks. Then, if things are going well, the brace is removed, but the boy faces another four to six weeks of limited leg use.

If the separation is great, surgical intervention is the treatment.

I can't be dogmatic about the diagnosis for your son. Other injuries cause similar pain. Stress fractures — tiny breaks in the bone — produce the same symptoms.

Dear Dr. Donohue • I have type 2 diabetes and take oral medicine for it, and I am on a diabetic diet. I haven't done much exercise since I was diagnosed. Would it be safe for me to exercise? — L.H.

Answer • Not only is exercise safe for you, you are encouraged to exercise regularly.

With type 2 diabetes, people often make enough insulin, but their body cells don't respond to it as they should. This is called insulin insensitivity. Blood sugar rises because of it. Fat plays a major role in insulin insensitivity. Reducing body fat increases cells' response to insulin. Exercise gets rid of body fat.

You do have to be careful about a blood-sugar drop when you start a program of exercise that you aren't used to. Carry sugar, raisins or a drink with carbohydrates with you. If you begin to sweat excessively or feel woozy, get some sugar in you quickly.

If you have your own unit for testing blood sugar, check your blood before, during and after you exercise. Do this a couple of times to find out what's going on with your sugar levels. You will have evidence if there's a dangerous drop in your sugar.

Write Dr. Donohue at P.O. Box 536475, Orlando, Fla. 32853-6475.