DEAR DR. DONOHUE: My daughter is 9 years old, 4 feet 10 inches tall and weighs 74 pounds. She plays fast-pitch softball and throws pretty hard. When is it safe for her to pitch windmills? Softball encourages windmill pitching. I was hoping to hold out until she is 12 or 13. – S.B.

ANSWER: A windmill pitch is one where the pitcher raises the throwing arm above the head, whips it around behind the body and releases the ball when arm and hand arrive at the normal position for an underhand throw — slightly in front of the body.

Such a pitch involves many muscles and joints. Leg, hip, back and abdominal muscles all get into the act. The greatest burden, however, is borne by the shoulder and arm muscles and the shoulder and elbow joints.

Nine years is young. At that age, bones and joints are immature. Bones have not yet calcified completely. They have sections of noncalcification — growth plates. The growth plates permit bones to elongate and thicken. They are vulnerable sites. Disturbing growth plates can lead to permanent disability.

I would not let her pitch in the windmill style until she is at least 13. You could teach her the mechanics and let her throw some balls to you, but that is the most stress that’s safe for her immature shoulder, arm and elbow. For college softball pitchers, there is an exercise protocol to follow to build strength for windmill pitching. It is not something that a 9-year-old ought to attempt.

Even when she reaches 13, the number of windmill pitches should be limited. At that age, she ought not to throw more than 15 such pitches in a game, and her weekly practice total of these pitches should not exceed 50.

DEAR DR. DONOHUE: I am a 74-year-old woman who has had a walking program for the past 10 years. I walk three times a week. My distance is three miles, and I complete those miles in one hour.

I wonder if I would derive more benefit by walking one mile in 15 minutes and repeating the mile walk for a total of three times, each walk taken at a different time of day. I think I might be better off walking faster for a shorter distance. Would I? – M.E.

ANSWER: There is no argument that a brisker walk burns more calories and gives the heart a more intense workout. You are currently walking one mile in 20 minutes. One mile in 15 minutes is more rigorous and benefits the heart somewhat more.

It’s perfectly OK to divide your walking into three sessions.

You don’t have to get too excited about this. You are doing fine as you are right now. If you want to introduce a little variety into your walk, change. Or alternate your program, one day using your tried-and-true method and the next day the new, faster program.

DEAR DR. DONOHUE: I would appreciate any help you can give me on how to improve my running speed. I’m not the slowest person in the world, but I am far from being the fastest. My coach tells me I need to run the bases faster to be a really good player. – H.J.

ANSWER: Two elements determine running speed: stride length and stride frequency. You have to experiment to find how much you can lengthen your stride without putting yourself off balance. You also have to experiment to find how much faster you can take strides without exhausting yourself.

Some years ago, in the Penn State Sports Medicine Newsletter, as it was then called, an article proposed a clever method of increasing stride length.

Set up a course with markers placed at increasingly greater distances. Put the first marker 16 inches from the start position. The second marker is put 20 inches from the first; the third marker, 24 inches from the second; the fourth, 28 inches from the third; and the fifth, 30 inches from the fourth. The idea is to take one stride between each marker.

It sounds to me like a good way to increase stride length with a precisely measured drill.

DEAR DR. DONOHUE: My 28-year-old son has been told he has juvenile diabetes. Juvenile? He’s 28.

Furthermore, I thought diabetes was inherited. There is not a single person in my family or my husband’s family who ever had diabetes.

Can you clarify? — K.S.

ANSWER: Juvenile diabetes is now called type 1 diabetes, since its onset is not limited to children. Its salient feature is the necessity to inject insulin to keep blood sugar in the normal range.

In type 1 diabetes, the pancreas, the site of insulin production, falters in its insulin manufacturing.

Genes have an influence on the genesis of type 1 (and type 2) diabetes, but they are not the entire answer. If one identical twin has type 1 diabetes, there is only a 50 percent chance that the other twin will come down with it. Identical twins have identical genes. If genes were the only factor influencing diabetes onset, both identical twins ought to have it.

It’s common for a person to have type 1 diabetes without any relative having had it.

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.