It floats around in the back of the mind, pricking the conscience of the sedentary and forcing healthy people to take action: Exercise.

But before jogging a mile, swimming a lap or pumping some iron, many experts first recommend that you assess your exercise capacity.

Health clubs now offer a wide range of fitness testing. Their pamphlets are slick, the instructors look qualified, the services seem impressive. But are consumers getting what they pay for?

Fitness tests are designed to learn a person's capacity for exercise. Results are used to tailor an exercise plan to each individual. But before undergoing one of these exams, experts advise asking a few questions.

What types of tests are given?

Most clubs will take a medical history and test for percentage of body fat, heart rate and blood pressure at rest, and muscle strength and endurance, said Maureen Smith, vice president of the Fitness Company, which operates the Westin Fitness Center on 24th and M streets NW in the District.. "If you have all these components, the price can range from $25 to $75 depending on who does the testing," she said.

Some clubs will include interpretation of the results in the fee. Others may charge more.

What do these tests reveal?

The tests show how fit a person is, not how healthy that individual is. They do not replace a physical examination by a doctor. "You can be very fit but still have cardiovascular disease," Smith said. "And that is where doctors become concerned."

One test monitors heart rate and blood pressure while a person is exercising, usually on a bicycle, but not exerting to maximum capacity. This test shows how well the body responds to exercise. A person may have a normal blood pressure reading while resting, but it may get abnormally high during exercise. Muscle endurance reveals how strong a person is and can be determined by lifting weights. Flexibility, which measures how well a person can stretch, can be tested by having a person sit with feet anchored and determining how easily he or she can touch the toes. Skin fold measurements indicate a person's fat content as opposed to muscle.

Stress tests, which measure a person's maximum exercise capability, show how the electrical activity of the heart responds to exercise. "You can identify the probability that an individual has heart disease from this exercise tolerance test," said James Graves, research associate with the Center for Exercise Science at the University of Florida. The cost can run from $100 to $400, depending on whether a physician is present or if there is medical interpretation done at the time.

Who benefits from testing?

People in their twenties without any risk factors for coronary disease, such as high blood pressure, smoking, family history of heart attacks or obesity, don't need one. Taking other fitness assessments before starting to exercise is optional. "I won't say it is not beneficial to have all these tests; any time you can be evaluated without risk, you get useful information," Graves said.

Some doctors think fitness testing is a good idea. "You are given an exercise prescription that puts you at minimal risk," said Dr. Pat Goreman, a cardiologist at George Washington University Medical Center. "If you have a person who wants to start jogging and does not have a fitness test done, you have too much enthusiasm with too little expertise and you end up with a lot of sore muscles and a person who is likely to lose interest."

For people in their thirties with one or more coronary risk factors, doctors suggest a clinical stress test run by a cardiologist. "You would get an indication of whether that person has normal responses to exercise," Goreman said.

People 45 and older should check with their doctor before having fitness tests done or starting an exercise program.

Are the instructors qualified?

Not long ago, the people developing exercise programs at health clubs were salespeople, not fitness experts, said the University of Florida's Graves. "We have come a long way," he said, "toward understanding that the prescription of exercise is an art and a science."

Fitness should be assessed by an exercise physiologist who can interpret results and advise what exercise program to start, how often and at what rate, said the Fitness Center's Smith.

The American College of Sports Medicine (ACSM) publishes guidelines and certifies exercise physiologists, who study the influence of exercise on body functions. Certification by ACSM requires ability to perform cardiopulmonary resuscitation and passing a written and practical examination. If they pass, they are certified as a "Health/Fitness Instructor," which means they can identify risk factors and conduct exercise tests and that they hold a bachelor's degree in a health-related field.

Not every person conducting fitness assessments is qualified or certified. One concern is that there are not enough people who know or understand the importance of certification, said Partlow. "We are trying for legislation to require licensing of fitness personnel, but it keeps getting tabled," she said. "One of the lobby groups against it is the club owners, because they don't want to pay more for more qualified staff."

Companies that insure individuals and facilities are now looking at whether personnel are certified, Partlow added.

Said Smith: "Find out the background of the person doing the testing. Your best bet is to go to a reputable program and ask for qualifications." Resources

The American College of Sports Medicine publishes pamphlets on exercise and health. Write: American College of Sports Medicine, Public Relations Dept., P.O. Box 1440, Indianapolis, Ind. 46206-1440.