You may be surprised to learn that your scale is an accurate gauge of neither fitness nor body composition, two factors which play a far more important role in your health than your weight. "The number on the scale is just a number," says Denise Bruner, M.D., a bariatric medicine specialist in private practice in Arlington, Virginia. For example, two people may both weigh the same amount, but they could be different heights and have different amounts of muscle and different bone densities. Their bodies could be polar opposites: One could be very fat, one could be very fit, she explains.

A slightly better fitness indicator is the Body Mass Index (BMI). You can figure your BMI by plugging your weight and height into a simple equation. The result can then be compared against a chart to determine whether your actual body mass is within a healthy range or if it indicates any level of obesity. For adults, that healthy range is between 19 and 25. However, the BMI measurement has its drawbacks, too. It presumes that you have the average muscle mass and bone density of a moderately active adult. Bruner cites baseball great Cal Ripken as an example of an extremely active person whose weight is misinterpreted by the BMI chart. According to the chart, Ripken, with a BMI of 30, is overweight. "The weight of his high muscle volume, not fat, puts him into the overweight category," explains Bruner, "but we already know he's fitter than most people on earth." Conversely, elderly adults with very low bone densities may appear to have a healthy BMI, when, in fact, much of their weight comes from excess fat rather than muscle or bone.

To get a clearer picture of that muscle-to-fat ratio, doctors and fitness professionals often use a skin caliper. This device pinches and measures the fat and skin on different parts of the body. After the measurements are taken, an average is calculated and the number is compared against a chart. Unfortunately, this method also has its drawbacks. People who want to get their body fat tested over the course of a fitness program may not get an accurate assessment of the changes in their bodies, since some testers pinch harder than others, and it's difficult to duplicate the exact locations where they pinched before.

According to Bruner, "a more sophisticated, reliable and reproducible test" is bioelectrical impedance or body composition analysis (BCA). This simple method involves painlessly sending a current through a person's arm and leg to get a body composition reading. The doctor or technician can then plug that reading into a computer program along with the subject's gender, height, weight and level of physical activity. Within moments the computer is able to provide the person's body fat percentage, muscle mass, bone density, estimated metabolic range and estimated ideal body weight. BCA is now widely available at many doctor's offices. Fitness centers also offer the test, sometimes at no additional charge.

So rather than focusing solely on your weight, assess your progress throughout a fitness program by getting an occasional BCA, suggests Bruner. But don't throw out your scale just yet. "Seeing changes on the scale each week can be very helpful and motivating," he says, "but people who exercise need to be mindful that they could be gaining muscle [if their scales don't reflect the weight loss they expect]. Since muscle is heavier than fat, the scale may not budge for a while, but clothes will fit better. So always take a full set of body measurements, too."

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