DEAR DR. DONOHUE: I am a 20-year-old college coed who has run daily for the past two years. It keeps me in shape, and it keeps my weight down. A couple of weeks ago, I started having pain in my right thigh, so I stopped running for a week, but the pain stayed on. I went to the health service at my school, and the doctor ordered an X-ray, which showed a stress fracture. How could this happen? I thought running strengthened bones. – M.A.

ANSWER: Exercise, including running, does strengthen bones. However, too much exercise with too little rest causes cracks in bones – stress fractures.

They’re common athletic injuries. They happen when excessive “stress” is put on bones without giving them enough time to repair the damages that stress imposes. Exercise is a stimulus to bone buildup. That buildup takes place during periods of rest. Rest is as essential to bone strength as exercise is.

Just about any bone can suffer a stress fracture, but the leg and foot bones are the ones most often involved. Army recruits used to suffer from foot stress fractures frequently, until the Army devised different training procedures. Ballet dancers who practice more than five hours a day are often victims of these fractures.

Two conditions dispose females to stress fractures. One is leg-length inequality, and the difference in leg length doesn’t have to be all that great.

The other condition is one in which a female athlete doesn’t take in enough calories to support her activities. Estrogen production wanes, and menstrual periods diminish or disappear completely. Calcium leaves bones. It’s premature osteoporosis. If you aren’t having normal periods, you should see a gynecologist. You might be suffering from this disorder.

It takes about six weeks for stress fractures to heal. Unless your doctor has insisted you do no exercise, you don’t have to be totally inactive. You can swim or bike to maintain your conditioning.

DEAR DR. DONOHUE: My son is a few inches shorter than the other kids his age. His mother and I are average height. He’s 9 years old, and he loves to play basketball. I don’t want to discourage him, but if he’s destined to be small, I’d like to interest him in another sport. Is there any way to estimate what his height will eventually be? – R.T.

ANSWER: Your son is a bit young to plan changes for his later years based on his present height. Most kids grow about an inch and three-quarters a year from age 3 to puberty, when a growth spurt takes place. Your son could shoot up then.

If you want an estimate of his adult height, here’s one way of going about it. Please take this for what it is: an estimate, a parlor game. Add five inches to his mother’s height. Then add that height to yours and divide the result by two. That’s an approximation of what a child’s adult height will be.

Don’t make any decisions now. If the boy is way off the charts for height based on his age, ask the family doctor if any interference is necessary.

DEAR DR. DONOHUE: This past summer I took up biking, and I really like it. I have lost 20 pounds, and I am not starving myself. In fact, I’m not dieting. I bike about 100 miles a week, sometimes more, sometimes less.

I don’t know if this is related to biking, but I have recently been having trouble with keeping an erection. Is the biking doing this? – T.P.

ANSWER: It could be. A hard bike seat can compress the nerves and the blood vessels that supply the penis. Erectile dysfunction is the result. Stop biking for a week and see what happens. If the dysfunction resolves, you’ll know it’s the seat’s fault. Get a cushioned one, one better suited to your anatomy.

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.