Inner Space

Can a know-it-all scale improve performance?

THE BIG IDEA: Tanita’s InnerScan ($120; www.tanita .com) is a scale on steroids: It not only reads your weight but employs a technology called “Bioelectrical Impedance Analysis,” or BIA, which zaps you with a painless electrical charge to compute body fat, bone mass, muscle mass, and five other metrics. The technology works because water, which conducts electricity, is distributed throughout mus-cles but not fat. Thus, measuring your body’s electrical resistance can instantly reveal your blubber-to-buff ratio. For endurance athletes, body fat and muscle mass are the numbers to watch. Overtrain and undereat and InnerScan will alert you to a resulting drop in muscle mass, a condition that reduces stamina and slows metabolism.

THE REALITY: I step directly onto InnerScan’s stainless-steel footpads to make contact with the BIA conductors. After a few seconds my weight flashes on the LCD screen: 166.2 pounds. Spot on. Next comes my body fat: 19.8 percent. Yikes! According to the Health Range Indicator chart shown in the InnerScan user guide, I’m bordering on “overfat,” only one level away from obese. Let’s be clear on this: I am not fat. As a manic health-food nut and adventure-sports junkie, I can assure you the wretched machine is broken. A second try using InnerScan’s “athlete” mode, which instructs the device to factor in more upper-body muscle, is far closer to reality (apparently the electrical jolt only gets as far as your belly button; from there a software algorithm estimates what lies above the belt). My body fat shrinks to 12.9 percent and my muscle mass swells to 137.8 pounds. Better yet, InnerScan says I have the metabolism of a 16-year-old. I’m 36.

I’m beginning to enjoy my InnerScan when Leonard Kaminsky, a professor of exercise science at Ball State University’s Human Performance Lab, informs me that BIA has a notoriously large margin of error when determining body fat. “It’s somewhere between 4 and 8 [percentage points],” he notes. Fortunately, however, there are ways to maximize precision. Heed the manufacturer’s advice: Stay hydrated and take readings at a set time each day, preferably in the evening. (When I tried InnerScan at daybreak, morning dehydration skewed my body fat toward pudgy). Also, engage the athlete setting if your workouts regularly total more than ten hours a week. Still worried about accuracy? Get a pricey ($250) but reliable DEXA (dual energy X-ray absortiometry) scan at your local hospital or imaging center, then use your BIA scale to track daily ups and downs.