DEAR DR. DONOHUE: My husband, a gym rat (his words, not mine), wants to start our 9-year-old son weightlifting. I think this is dangerous and foolish. As far as I can tell, the boy hasn’t begun puberty, and I know testosterone is necessary for muscle development. I also wonder about the damage to bones and muscles from too early a start in weightlifting. Please tell me your thoughts. And when does puberty begin? — A.L.

ANSWER: In the past, the thoughts you express were common teachings. Weightlifting was discouraged until a child had entered puberty. The belief was that without testosterone, muscle and strength building were impossible. That’s not the case, and it’s been shown not to be true time and again.

For a boy his age, however, aerobic exercise should be emphasized, things like running, jogging, biking, swimming and playing sports such as soccer, where there is plenty of nonstop running.

As for strength training, go for it. The boy has a knowledgeable instructor in his dad. The child isn’t going to turn into a Hercules, but his muscles and bones are going to get stronger. He will improve his balance and agility and his confidence. His training will prevent injuries.

Your husband has to go about this sensibly. The boy shouldn’t lift any weight he cannot lift easily for eight successive times. That’s the best way to determine a safe weight. In the early days of his program, he should lift the weight only six times and then take a rest. He can do two more sets of six lifts if he’s not straining to do so.

When he can lift a weight 16 times successively, your husband can increase the weight, and the boy goes back to six consecutive lifts

He also can do exercises that use body weight as the resistance — sit-ups, push-ups, chin-ups and on and on.

The average age for the onset of male puberty is 9 years, 6 months.

DEAR DR. DONOHUE: I have a 14-year-old daughter who is a runner. She developed a painful lump beneath her knee. She always has been a stoic about any injuries and pain, but she complained about this, so I knew it was troublesome. I took her to our family doctor. He made the diagnosis of Osgood-Schlatter’s disease. We’re in the dark about it. What is it? How long will she be off running? — L.H.

ANSWER: Feel right beneath your kneecap. You’ll feel a bony projection that comes from the tibia, the larger of the two lower leg bones. The projection is the tibial tubercle. It’s not always firmly attached to the tibia until maturity is attained.

The tendon of the large thigh muscles attaches to that tubercle. When those powerful thigh muscles contract, the tendon pulls on the tubercle with great force. Repeated contractions can pull it loose from the bone.

The result is a painful, swollen lump — Osgood-Schlatter’s disease. It’s a common childhood injury.

Rest and restricted activities almost always heal the condition. Movement isn’t totally forbidden, so long as it doesn’t cause pain. It takes around two months for healing. In some instances, it can take as long as a year or more.

DEAR DR. DONOHUE: Please explain what eccentric contractions are. I understand they are most important in muscle development. I’m starting a muscle-building program, and I’d like to incorporate eccentric contractions into my program. — B.D.

ANSWER: During an eccentric contraction, the muscle is lengthening.

When you do a biceps curl, you lift the barbell from waist to shoulder level by contracting the biceps muscle, the muscle on the front of the upper arm. It bulges.

When you lower the weight back down to the waist, the biceps lengthens. It is contracting eccentrically. Eccentric contractions are the more important muscle stressor. You should not let the weight drop quickly. You should fight gravity by lowering it slowly. You’ll find that muscle growth increases faster when you pay attention to eccentric muscle contractions.

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.