I’ve started working out with a personal trainer, and he suggested that I get a body composition test before starting my exercise program. Is this just a gimmick or a good idea?


It’s a great idea. Assuming that you will be doing cardiovascular exercise as well as strength training, knowing your muscle mass and body fat percentages is extremely helpful for setting your exercise goals and tracking your progress.

You probably know that you lose muscle mass as you age. Starting in your 30s, you lose roughly 3% to 8% each decade, but most people don’t realize that the rate of muscle loss accelerates after age 60. Muscle loss, known as sarcopenia, is one of the major contributors to infirmity in one’s later years. But the good news is, it’s not inevitable. Strength training can significantly mitigate age-related muscle loss.

For this reason, preserving—and even building—lean muscle is extremely important as you grow older. Building muscle improves how you perform your activities of daily living (everything from bathing to getting in and out of bed) and boosts your metabolism, burning additional calories all day long and helping you to maintain a healthy body weight.

A traditional scale measures only your weight—it doesn’t differentiate between muscle mass and fat mass. This means you could work out with your trainer for a few months, losing five pounds of body fat while adding five pounds of new muscle mass. A traditional scale, however, would show no change when, in fact, you would have achieved significant results!

There are numerous ways to test your body composition, but the most accurate methods are hydrostatic weighing and the dual-energy X-ray absorptiometry (DEXA) scan, which is commonly used to evaluate bone mineral density to determine a person’s risk for osteoporosis. Hydrostatic weighing, which is done completely underwater, is usually available only in research settings. While a DEXA scan performed to check your bone density typically includes only the hip and spine, the entire body is scanned to measure body composition. The DEXA scan for body composition, which is paid out-of-pocket, is available in university settings for about $60 and $150 to $200 at private facilities. To find location near you where you can get a DEXA scan to check your body composition, go to this website. If you get this scan, ask your doctor to help you interpret the results.

Another option, known as bioelectrical impedance analysis, uses a scale to measure resistance to an electrical current as it passes through your body while standing. This method is not as accurate as the DEXA and underwater methods, but it can be an affordable and convenient way to assess your body fat and muscle mass percentages. This type of testing is becoming increasingly available at many gyms and usually costs less than $50.

Skin calipers are yet another method. This is a handheld device that’s used to pinch areas of skin to estimate body fat. This type of testing, known as skinfold measurement, is fast and widely available at gyms but not as accurate as the methods described above—often because the person administering the test is not properly trained. For example, the standard error is 3.5%, depending on the equation applied, compared with 2.7% error for a hydrostatic measurement, according to the American Council on Exercise.

Skinfold measurements should be taken at various areas of the body to help ensure that you’re getting the most accurate estimate. After about six weeks of starting a new exercise program, some people use skinfold testing to see if their body composition is changing. Because regular exercise helps increase lean muscle mass, which may translate into more body weight, skinfold testing can give you a reasonable estimate of any improvements in your body fat percentage. This information will go a long way toward keeping you motivated!

Whatever method you choose, be sure to review the results of your body composition test with your personal trainer so that you can work together to design an exercise regimen with attainable goals. If you’re tested in a gym, your trainer can tell you what the optimal body fat and muscle mass ranges are for your age.