When Sam teaches the students in his aerobics class, he works to get them in good-enough shape to exercise at their target heart rate -- intense enough to make them fit yet easy enough to help protect them from injury.
To become fit, you need to increase your heart rate by more than 20 beats a minute above its rate at rest.
Obviously, exercising at an intensity greater than that will cause an even greater improvement in heart function, but it could also increase your chance of injury. To be safe, your target heart rate should be 60 percent of your maximum heart rate.
Your maximum heart rate is the fastest your heart can beat and still pump blood through your body. Maximum heart rate is usually 220 minus your age. As you age, your heart weakens and your maximum heart rate decreases. A 50-year-old has a maximum heart rate of 170 and a target heart rate of 102 (60 percent of 170).
You don't need to take your pulse to measure exercise intensity. You will know you have reached your target heart rate when you begin to raise your shoulders and breathe faster to get more air. You should still be able to talk.
* Q: Is it OK to exercise when I have a cold? I don't want to stay inactive so long that I get out of condition.
A: It is probably all right to exercise when you have a cold, provided you don't have a fever and your muscles don't hurt.
There are no controlled scientific studies showing that people who exercise when they have colds harm themselves. The odds are that exercising when you have a cold -- but no fever -- is usually harmless; it is likely that serious reactions would be reported in the medical literature. Such reports haven't materialized.
On the other hand, when you have a fever, your heart has to work much harder to pump extra blood from your internal organs to your skin, where excess body heat can be released into the surrounding air. (This is the process that helps keep your temperature from rising too high.)
Your heart has to supply your muscles with oxygen and energy. Some viruses that infect your nose and throat can also infect your heart muscle. The combination of the extra work and infected heart muscle can cause an irregular heart beat.
When your muscles are affected by a respiratory virus, they hurt during exercise. Exercising when your muscles hurt can lead to injury. Infected muscles also are weaker and can't be worked as long; the virus can reduce the concentration of enzymes that helps muscle tissue process and use oxygen.
If you have a cold and feel good enough to exercise, you probably won't hurt yourself. If, however, you have a fever or aching muscles, you are better off resting. You won't lose much conditioning unless you stop exercising for more than a week.
Q: Why is vitamin D added to milk?
A: Most of us don't get enough vitamin D in the foods we eat, making a lack of vitamin D the only common vitamin deficiency in the United States today. That's why vitamin D is often added to milk.
You need vitamin D for strong bones. One-third of the people who suffer from hip fractures are deficient in vitamin D. Too little vitamin D also may increase chances of developing cancer of the prostate and breast.
Your body can make its own vitamin D if you expose a few inches of skin to sunlight for just 10 minutes a day. But many people especially the elderly develop a vitamin-D deficiency because they don't go outdoors very often, while concern about developing skin cancer has caused many others to avoid sunlight altogether.
You can also get vitamin D by eating fatty fish (sardines, salmon and tuna), eggs, liver and vitamin D-fortified milk and butter. But concern about heart attacks has caused many people to reduce their intake of these foods.
To find out if you are low in vitamin D, ask your doctor to test your blood for 24 hydroxy vitamin D. If you have low levels, you need more vitamin D. Drink vitamin D-fortified skim milk or take a supplement containing 400 international units of vitamin D. And get a little sun.
Dr. Mirkin is a practicing physician in Silver Spring specializing in sports medicine and nutrition.
United Feature Syndicate