Muscle is obviously important to gym rats and pro athletes. But even if getting "swole" isn't on your list of life goals, there are good reasons to keep track of your muscle mass.
The amount of muscle you have affects everything from the number you see on the scale to your ability to lift a bag of groceries to the recommended number of calories you should eat daily.
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Muscle is one of the main components in the human body, alongside fat, bones and water. Your organs are also made of muscle (the heart, for example, is comprised of specialized cardiac muscle fibers), but organs account for a very small amount of total muscle mass — less than 10 pounds for the average man, according to a December 2012 study in the American Journal of Forensic Medical Pathology. When we talk about muscle, we're primarily talking about the soft tissue that facilitates movement in the human body.
Muscle is dense. A pound of muscle is about 20 percent smaller by volume than a pound of fat — which means that two people of identical height and weight may look very different if one has a high amount of muscle mass and the other has a large amount of fat. The leaner individual may appear smaller because her muscle mass is more compact.
There is no rule of thumb when it comes to how much of a person's total mass is muscle. Muscle mass varies based on factors such as height, age, weight, sex, exercise habits and more.
How to Measure Muscle Mass
Muscle mass is usually expressed as a percentage of total body composition. But until recently, scientists had no easy way to measure muscle mass. Instead, they estimated the amount of fat in the body and subtracted that figure from total mass to calculate the remaining "fat-free mass."
A variety of methods have been used to measure body fat, including techniques that involved skin-fold calipers and underwater weighing. Such measurements were eventually used to develop the Body Mass Index (BMI), a popular formula that estimates body fat percentages based on sex, height and weight.
None of these methods, however, can render a precise measurement of muscle mass, University of Minnesota exercise physiologist Donald Dengel, PhD, tells LIVESTRONG.com. That's because fat-free mass also includes bone mass, which differs by individual.
In the mid-1990s, exercise scientists began to use a technique called dual-energy X-ray absorptiometry, or DEXA scans, to measure body composition. These measurements can differentiate between fat mass, bone mass and lean soft tissue, providing the most accurate measure of muscle mass we have today. DEXA scans can even identify the exact location and density of fat deposits, muscle and bones — but they require a trip to a health care facility and can cost $150 or more.
What Your Muscles Have to Do With Your Calorie Intake
“Muscle, even when it’s not being used, controls the number of calories you burn in a day,” Dengel says. “It’s directly correlated to energy expenditure. The more muscle you have, the more calories you’ll need.”
Why You Might Want to Measure Muscle
For most of us, muscle mass measurements are primarily a way to help us understand if we're progressing in our exercise and weight-lifting routines. A bodybuilding coach, for example, would hope to see significant muscle growth over the course of a training season.
A physician, on the other hand, might use muscle mass measurements to determine whether an aging patient is losing muscle more rapidly than is typical. (Without exercise, the human body typically begins losing muscle around age 30, a process known as sarcopenia that accelerates with age, according to the International Osteoporosis Foundation.)
DEXA scans can also be used to identify imbalances in muscle mass. Asymmetry in muscle mass is common, especially in athletes. You'd expect a pitcher to have a stronger throwing arm, right? A September 2019 study of college softball players in the International Journal of Sports Medicine found that players' throwing arms often had significantly greater lean muscle mass than their non-throwing arms — no matter their position on the team.
Some muscle imbalances can lead to injury, Dengel says, so strength coaches often work to even out asymmetry, adding single-leg or single-arm exercises to a training routine in the hopes of strengthening those weaker muscles.
If a runner, for instance, has more muscle in one leg than another, he is likely to compensate for the imbalance in ways that may ultimately lead to harm. "The human body wants to be in symmetrical balance," Dengel says. "The more asymmetrical you are, the more likely you'll get injured."
- International Journal of Sports Medicine: "Total and Regional Body Composition of NCAA Division I Collegiate Female Softball Athletes"
- American Journal of Forensic Medical Pathology: "Normal Organ Weights in Men: Part II-the Brain, Lungs, Liver, Spleen, and Kidneys"
- International Osteoporosis Foundation: "What Is Sarcopenia?"