DEAR DR. DONOHUE: I was talking to my husband about us getting into shape. We do hardly any exercise. When I was younger, I loved jumping rope. I didn’t believe it, but he was actually receptive to rope jumping. He did some amateur boxing, and he said rope jumping got him into better shape than anything else. What do you think of this as an exercise? – M.M.

ANSWER: Rope jumping is as good an exercise as anyone can do for promoting heart health. You might have fond memories of doing it as a child, but it’s a very demanding exercise. A person weighing 140 pounds who jumps 120 times a minute – a pace that only those who are extremely well-conditioned can maintain – burns 12 calories a minute. It used to be said that 10 minutes of rope jumping equals 30 minutes of running, but that isn’t true.

Not only does skipping rope improve cardiac performance, it develops coordination and agility. It strengthens leg muscles and upper-body muscles too.

Some things are important to keep in mind so that rope jumping is a doable exercise without injury. One is the length of the rope. Put your feet on the center of the rope and draw the ends upward. The right-size rope should reach to just under the armpits. The jump should be only high enough to clear the rope. Too high a jump stresses the knees. Land on the balls of your feet with the knees slightly bent. Wear shoes that cushion your landing. If possible, jump on a giving surface, like a wooden floor or the lawn.

Start out with a very modest time of continued jumping, one or two minutes. Much more will leave you breathless. Gradually increase the time. After two months, you’ll be able to do 15 minutes at a stretch. In the beginning, turn the rope about 70 times a minute, a little more than once a second. While you’re increasing the time of jumping, simultaneously increase the speed of rope turning.

You’ve picked a good exercise. You’ve also picked a difficult exercise.

DEAR DR. DONOHUE: I am a male dancer, and I am 22. In the past two years, I have had back pain that has kept me from dancing for three weeks at a time. I’ve been examined and X-rayed, and nothing has been found. What could I be doing wrong? I do a lot of lifting of my partner, and I think that’s what sets my back off. – J.W.

ANSWER: Back pain from lifting a partner is a common dancing injury. Usually it happens because the lifter’s lower back curves too far inward. You have to lift with your back as straight as possible – no inward sway.

Exercises for the lower back and the abdominal muscles will help you. And you have to consciously straighten your lower back on every lift.

You realize the causes of back pain are many. If your back kicks up again, have another examination.

DEAR DR. DONOHUE: I have been jogging for one whole year. I’d like to know if I am in better shape than I was before I started. I don’t feel all that different. How do I test myself? I am a 44-year-old female. – L.P.

ANSWER: Dr. James Rippe at Tufts University has a test for you. If a woman your age can jog or fast walk a mile in less than 14 minutes and 12 seconds, she’s in excellent health.

If you don’t have a mile track, most high schools have a quarter-mile track you can use.

DEAR DR. DONOHUE: I just got through reading your article advising a weightlifter to rest muscles for 24 hours between sessions. I teach six 45-minute periods of physical education for kids in kindergarten through fourth grade four days a week. By Wednesday, my body struggles to demonstrate the warm-ups and cool-downs. Is my problem a lack of 24-hour rest or is it my 56 years? – N.D.

ANSWER: The 24-hour rest is for weightlifting, and it’s directed to the same muscles. It’s OK to weightlift every day if you exercise different muscles. Aerobic exercise – your exercise – can be a daily affair. It shouldn’t be a struggle. Can you pace yourself better? Let some of your demonstration be in words, not in deeds.

DEAR DR. DONOHUE: Can vitamin D be gotten from sunshine passing through car-window and house-window glass? – N.H.

ANSWER: No. Window glass filters out ultraviolet B rays, the ones responsible for the conversion of a skin material into the vitamin.

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.