How to Measure Muscle & Bone Mass

One way to determine if you're fit or fat is by measuring your ratio of lean muscle and bone mass to body fat. Some relatively simple methods you can do at home with only a tape measure or calculator, but others require specialized testing equipment at a fitness facility, research setting or doctor's office. Knowing the amount of muscle and bone mass in your body can help you better gauge your progress while losing weight and getting in shape.

The body mass index, or BMI, is used as a quick way of determining how much body fat you're carrying around. Hundreds of studies have linked a high BMI not just to obesity, but to a greater risk for chronic disease and mortality. The main drawback with the BMI is that it can't distinguish between body fat and lean body mass, especially in bodybuilders. The formula is weight in pounds divided by height in inches, squared, multiplied by 703. For a 150-pound woman who is 5 feet, 5 inches tall, the equation would be [150 ÷ (65 x 65)] x 703 = 24.96. A healthy BMI is between 18.5 and 24.9; overweight is between 25 and 29.9; obese is a BMI of 30 or greater.

Bioelectric impedance, or BIA, sends a small, safe electric current through your body to measure the resistance, because there's more resistance through body fat than lean body mass and water. Professional BIA systems use electrodes attached to a machine, but home versions send the current through your bare feet when you step on a scale. A readout gives your total weight, water content, fat and bone and muscle mass. However, the ratio of body water to fat may be change during illness, dehydration or weight loss, thus decreasing the reading's accuracy. Plus, BIA isn't as effective if your BMI is 35 or higher.

Underwater weighing, or densitometry, is often used in research settings to measure muscle and body mass. You lie submerged in a tank of water while the attendant estimates body volume, body density, and body fat percentage, since fat is more buoyant than water. An alternative is air-displacement plethysmography, where you sit in a small chamber of water while wearing a bathing suit. With something known as the dilution method, or hydrometry, you would drink an isotope-labeled water and give body fluid samples that are analyzed for total body water, fat-free body mass and body fat mass. Dilution is the only technique that is suitable if your BMI is above 40.

Imagining techniques are the most accurate of all, but they are usually only available in hospitals or research settings and are quite expensive. The dual energy X-ray absorptiometry scan, or DEXA, uses two low-level X-ray beams to estimate fat-free mass, fat mass and bone mineral density. Computerized tomography, or CT, and magnetic resonance imaging, or MRI, are the most accurate scans of all, but they can't accommodate people with a BMI of 35 or higher and should not be used if you're pregnant.