How to Calculate Muscle Mass Percentage

Diet, exercise and genetics affects your lean muscle mass.
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Special scales, MRIs, formulas, and BMI and muscle mass calculators: These are the types of methods that can be used to calculate lean body mass and muscle mass percentage.

BMI and Muscle Mass Percentage

Your body mass index (BMI) is a measure of weight, calculated as weight divided by height. According to The Department of Health and Human Services, BMI measures excess weight rather than excess fat. It should be noted that BMI does not distinguish between excess fat, muscle or bone mass and does not provide the distribution of fat.

Overall, your body mass is composed of body fat and lean body mass, and BMI does not differentiate the two, explains a paper in Nutrition Today. In other words, someone can have a high BMI but low body fat mass, or a high BMI and low lean body mass.

It should be noted that lean body mass is not made up only of muscle, but of bone as well.

Read more: BMI vs. Body Fat Percentage

Methods of Calculating Muscle Mass

There are a few methods that can be used to measure muscle mass, though, in some instances, accuracy is questionable. Methods include:

MRI: MRI is probably the most reliable method of calculating muscle mass percentage. In one July 2000 study in The Journal of Applied Physiology (which has since been reviewed and corrected in 2014), researchers used MRIs to examine the influence that age, body weight, height and gender had on skeletal muscle mass distribution. With the MRI, researchers were able to get an accurate reading of muscle mass, and found, among other things, a reduction in skeletal muscle mass began in the third decade, and men had significantly more skeletal muscle mass.

Scale: One way to measure lean body mass, according to the University of California, Berkeley, is to use a body fat scale, which uses bioelectrical impedance technology to measure your body fat. Once the body fat has been calculated, you can subtract that number from 100 to get your lean body mass. The only downside of using a body fat scale is that they're often inaccurate and several variables can affect your results.

Formula: The Naval Physical Readiness Program recommends using a formula that measures the circumference of different body parts to measure your circumference value (CV), which are then placed on a chart with body fat percentage estimates. You can then use that number to calculate lean body mass percentage. While this method of calculating body fat and lean body mass percentage is used by the military, it's not completely accurate.

Read more: Define Lean Muscle

Reasons for Low Muscle Mass

Low muscle mass has recently been included in the diagnostic criteria for malnutrition, explains a January 2019 paper in The Journal of Post-Acute and Long-Term Care Medicine.

Other factors that play a role in muscle mass development are age and diet. Protein is the prime muscle food and will aid in muscle growth, according to Harvard Health Publishing.

As far as age, after 30 you begin to lose as much as 3 to 5 percent of your muscle mass per decade. Men will lose up to 30 percent of their muscle mass in their lifetime.