DEAR DR. DONOHUE: I am in high school and want to build my strength and endurance. Would a daily workout on an elliptical machine or a treadmill help me more? – A. Student

ANSWER: You have asked more than you bargained for. For readers, an elliptical machine takes up as much space as a treadmill and comes in many shapes and designs. All ellipticals have one thing in common: The exerciser’s feet never leave the surface, be it a pedal, a step or a small platform. On an elliptical machine a person is able to go forward or backward. Since the feet don’t come up in the air and then land on a hard surface, elliptical exercise is low-impact exercise, the kind of exercise that’s easy on joints.

Both elliptical machines and treadmills are aerobic exercise, exercise in which large muscles (here the legs) are in uninterrupted motion for a prolonged period. This is the kind of exercise that’s good for the heart. It’s equivalent to brisk walking and running. Both a treadmill and an elliptical machine give good endurance workouts. If you have access to both, alternate their use. It adds variety to your program. If you have to make a choice, choose the one that appeals more to you. You can’t go wrong with either.

The hidden element to your question is this: Do endurance exercise and strength exercise work against each other? When strength-trained athletes take up endurance exercise like long-distance running, many of them notice their muscles are losing size. Endurance training taxes muscles that are different from the muscles used in strength training. This is a point of interest only to those who are strictly strength athletes or strictly endurance athletes.

Most other athletes benefit from combining strength training with endurance training. Endurance training alone – running, using a treadmill or an elliptical machine – won’t build super strength. The only way to attain that is through resistance exercise – weightlifting.

DEAR DR. DONOHUE: I am a 70-year-old skier. Ten years ago I was diagnosed with early emphysema. No surprise. I had smoked a pack of cigarettes a day for 40 years. I now walk two miles a day and am in very good shape. I have no trouble skiing in the East at 3,000- to 4,000-feet elevation. However, nine years ago, while skiing in the Rockies, I could ski only 400 yards before I became seriously winded. I have been offered a virtually free trip to the Rockies this year. I am afraid my wind won’t measure up. Is there anything I can do to improve my breathing at high altitudes? – F.A.

ANSWER: Your doctor has to quantify how bad your lungs are. Through lung-function tests, the doctor can answer this question for you and can give you assurance that you are up to high-altitude exercise.

Most people can tolerate an altitude of 8,000 feet without much trouble. A person with emphysema might run into trouble around 5,000 feet.

If you take your time in reaching higher altitudes, you give your body a chance to adjust to the lower oxygen pressure. If you find yourself breathless at a certain height, drop down 1,000 feet and stay there at least a full day. When you ascend to the height that gave you trouble, spend a day there but descend to spend the night at a lower level. If you do this in steps, you should reach the height where you want to ski.

Keep yourself hydrated. It’s easy to become dry at heights, and that affects your breathing.

Medicines might help you. Diamox is often used to help people adapt to altitudes they are not used to. It’s a mild water pill. And sildenafil, the drug for erectile dysfunction, also works as an altitude adapter. You’ll have to ask your doctor if they’re safe for you. Both require a prescription.

DEAR DR. DONOHUE: My grandson is 9 and started running 1-mile races two years ago. It was fun for him for a while. Last year, he ran a few 3-mile races, and now his father has him entered in 3-mile races almost every weekend. He also runs 3-mile practices during the week. Should he be doing all this running at his age? He would like to play soccer, but his parents don’t want to take the time for that. – A.J.

ANSWER: That’s far too much running for immature joints and bones. He’s headed for an overuse injury. He’s also headed for burnout. His parents should let him choose the sport he wishes to participate in.

DEAR DR. DONOHUE: I would like to know if there is a physiatrist in my area. I read about this kind of doctor in your article. – E.L.

ANSWER: If ever I was sorry about using a word, I am sorry about using physiatrist (fizz-EYE-uh-tryst or fizz-ee-AT-rist). I threw an unintentional curve ball.

The better understood word is physical-medicine doctor or rehabilitation doctor. Nearly every hospital has one or more of these specialists.

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