Knowing your body mass index is useful, but BMI, like body weight alone, doesn't pinpoint the exact composition of your body. (To learn more about BMI, go to: www.fasterbetterstronger.com.) A body-composition test will give you your ratio of fat to total body mass. Women typically have a much higher ratio than men. The minimum considered safe is 5 percent for males and 12 percent for females. A healthy body-fat composition is 13-18 percent for men, and 20-24 percent for women.
1. Bioimpedance is now a feature on many home bathroom scales. This technology gauges your body composition by measuring electrical signals as they pass through fat and lean mass. Fat offers higher internal resistance than lean body mass; likewise, if you're dehydrated, your body will offer more resistance to the current. To ensure the greatest accuracy in tracking your progress, always drink eight ounces of water 15 minutes before stepping on the scale, and weigh yourself at the same time every day and under the same circumstances (for example, just after rising instead of after a number of different workouts).
2. Skin fold tests are basically the pinch test, using calipers to precisely measure the amount pinched. You'll find these tests offered at performance labs, gyms, spas and health clubs. They are simple and quick, but accuracy depends upon the tester's experience and the algorithm they use. However, if the same person tests you over time, you will have "consistent inconsistencies," and you can easily and inexpensively monitor your progress.
3. Hydrostatic weighing uses your underwater weight to calculate your body density and, from that number, your body-fat percentage. It has a low margin of error and can be found at many university health centers.
4. BOD POD uses air displacement to very accurately measure your fat percentage as well as your resting metabolic rate (how many calories your body burns in a resting condition). You can probably find a lab or medical facility that uses this technology near you. It is a little more expensive than the methods previously mentioned, but it's more precise. You can find this technology at many universities, labs and medical facilities.
5. Dual-Energy X-Ray Absorptiometry (DEXA) is a technology that differentiates lean mass from fat mass from bone, based on how these tissues absorb two levels of X-ray differently. It's very precise and reliable and can provide your total body-fat composition as well as bone mineral density. It will even pinpoint the location of fat deposits. DEXA is considered the gold standard; the latest body-composition research uses this method. Check with your doctor for an OK and with your insurance provider (it may be covered). Many university health systems offer DEXA.
By testing your body-fat composition before beginning an exercise program and as you progress, you will know when you are losing fat and gaining lean body mass as you exercise, and that fuels motivation. Just don't test it too often, since body-fat measurements don't show small changes. Every 4-8 weeks is optimal.
( Eric Heiden, M.D., a five-time Olympic gold medalist speed skater, is now an orthopedic surgeon in Utah. He co-authored "Faster, Better, Stronger: Your Fitness Bible" (HarperCollins) with exercise performance physician Max Testa, M.D., and DeAnne Musolf. Visit www.fasterbetterstronger.com.)