How Much of Your Body Mass Is Actually Muscle?

If you're working out to build muscle, don't be upset if you don't like the number you see on the scale. While it may tell you how much you weigh, it doesn't tell you how much of that weight is muscle vs. fat. When it comes to health, body composition counts. Determining exactly how much muscle you have, however, is not as easy as it sounds.

A woman has less muscle mass than a man.
Credit: Boarding1Now/iStock/Getty Images

Body Composition

Body composition includes more than just muscle and fat; it also includes bones, organs, tendons and ligaments. Healthwise, body composition typically measures body fat vs. lean body mass. Lean body mass not only includes your muscle but also organs, bones, tendons and ligaments. There are a number of tools used to measure body fat, including calipers, bioelectric impedance and underwater weighing. But according to Steven Heymsfield, author of "Human Body Composition," muscle mass testing is limited and based on cadaver dissection studies. While you may be able to determine your exact body fat percentage, determining your muscle mass is a little less precise.

Estimating Muscle Mass

When determining muscle mass, you want to know how much skeletal muscle you have, and this includes those muscles you're working out at the gym. According to Heymsfield, 30 to 40 percent of a healthy person's body mass is made up of skeletal muscle. A study from 2000 published in the Journal of Applied Physiology found through whole body MRI testing that women tend to have less muscle mass, closer to 30 percent of their body weights, than men, who have closer to 40 percent. Based on this information, a 200-pound man has about 80 pounds of muscle mass.

Muscle mass is not the same for everyone, however, and may vary depending on age and fitness level. The amount of muscle you have in your body, according to the authors of the 2000 study in the Journal of Applied Physiology, decreases as you age and dramatically after age 45. And according to a classic study in the Journal of Sports Sciences, fitness and type of activity make a difference in muscle mass, with bodybuilders having a greater percentage of muscle than endurance athletes.

Use Body Fat Percentage

Researchers have not yet figured out how to determine muscle mass without the use of technologies such as an MRI, but you may be able to calculate your lean body mass -- which includes muscle mass -- using body fat percentage. Typically, testing by a professional is necessary for measuring body fat, but the Navy has developed a mathematical formula that uses body measurements to estimate body fat.

The formula differs for men and women. To determine body fat percentage in men: [86.010 x log10(waist - neck)] - [70.041 x log10(height)] + 36.76. And in women: [163.205 x log10(waist + hip - neck)] - [97.684 x log10(height)] - 78.387. All measurements are in inches.

For example, a 6-foot-tall man with a neck circumference of 14 inches and waist circumference 36 inches has a body fat percentage of 22 percent.

According to the National Academy of Sports Medicine, you can determine lean body mass by subtracting your body fat percentage from 100. The 6-foot-tall man has 78 percent lean body mass, and because he weighs 200 pounds, he has 156 pounds of lean body mass. Calculating lean body mass from body fat percentage doesn't give a specific amount for muscle mass alone, however.

Benefits of Knowing Body Fat

While you might be interested in knowing how much of your weight is rock-hard muscle, body fat gives you a better picture of your health. A high percentage of body fat increases your risk of chronic health conditions such as heart disease, diabetes and high blood pressure. A good percentage of body fat ranges from 11 to 14 percent for men and 16 to 23 percent for women. An acceptable percentage of body fat ranges from 15 to 20 percent for men and 24 to 30 percent for women. You are considered overweight if your body fat ranges from 21 to 24 percent for men and 31 to 36 percent for women -- and obese with percentages greater than these numbers. According to Human Kinetics, women have a higher percentage of body fat due to hormones and the need for stored energy for childbearing.

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