IN RECENT YEARS everyone has become aware of the increased interest in aerobics. But just what is aerobics?

An aerobic workout conditions the cardiovascular system. Cardiovascular endurance is the most crucial element of fitness. The stronger the heart, the better the cardiovascular system delivers oxygen throughout the body. Many forms of exercise qualify as aerobic, such as: brisk walking or jogging, distance running, cross-country skiing, swimming, rowing, cycling and jumping rope.

To understand what you must do to get a proper aerobic workout and benefit from these activities, remember the acronym FIT.

Frequency: How often? At least three times per week.

Intensity: How hard? Hard enough to get your heart rate into your "target zone" (see below).

Time: How long? At least 20 minutes.

You must exercise aerobically at least three times per week for 20 minutes with your heart rate in the target zone. (Your workout should also include, of course, a five-minute warmup and a five-minute cool-down). Less than this will not help you achieve an adequate training effect (although any exercise is always better than none).

Monitoring your heart rate is a valuable way to measure exercise intensity and to gauge improvement in an aerobic program. The target heart rate is extremely important to achieve a safe, effective fitness program. A target rate of safe intensity can be determined as follows:

Take 220 and subtract your age. This will give you your theoretical maximum heart rate.

Now find your resting heart rate (you should take it upon awakening in the morning, and take your resting heart rate several times during a week and average them).

Next, select an intensity level. If you are in poor/fair condition, you should use 60 percent; average condition, 70 percent; and good/excellent condition, 80 percent.

Then, subtract your resting heart rate from your maximum heart rate. Multiply that by the percent representing your selected intensity level and add your resting heart rate.

This is your target heart rate.

For example, Bill is age 40, in average condition with a resting heart rate of 70 beats per minute. Bill's target heart rate is figured as follows:

220 minus 40 (his age) equals 180 (his maximum heart rate).

180 minus 70 (his resting heart rate) equals 110.

110 times .7 equals 77. Then, add 70 to get 147, his target heart rate.

Bill's goal is thus to exercise hard enough to sustain a heart rate of 147 beats per minute for at least 20 minutes three times per week. Remember to stay close to your target heart rate. If you are over it, you are working too hard and should slow down. If you are much below your target, you are not working hard enough to exert a conditioning effect on your heart. Try a little harder.

Check your heart rate when you exercise by pausing to take your pulse. The best spot to measure your pulse is at the radial artery in your wrist. You can also take your pulse at the carotid artery in the neck, although some studies have shown this to be less accurate. Find your pulse with your index and middle fingers and count the beat for 10 seconds, counting the first beat as zero. Then multiply by six to get your rate per minute. Be patient. It takes time to learn to take a pulse reliably.

Check your pulse frequently, about every five minutes. After you have been exercising regularly for a while, you will learn to judge when you are in the target zone so you can take your pulse less frequently.

A fit heart pumps more blood with each beat than an unfit heart and, thus, needs to beat fewer times to deliver the oxygen and nutrients the body requires. People who are fit tend to have low resting heart rates. In fact, the hearts of many endurance athletes beat fewer than 50 times per minute.

So get moving, get an aerobic workout three times per week and you're on your way to getting FIT.