What skeletal muscle mass means
Skeletal muscle mass is the total weight of all the skeletal muscles in your body: the voluntary muscle attached to your skeleton that moves you, holds your posture, and lifts things. It is reported in pounds or kilograms, and often as a percentage of your body weight.
Your body has three kinds of muscle. Skeletal muscle is the one you can see and train. Smooth muscle lines your organs and blood vessels. Cardiac muscle is your heart. Only the first responds to resistance training, which is why it is the one body composition tools single out and the one worth paying attention to. Different Health measures skeletal muscle mass in-lab with InBody, alongside body fat and visceral fat, as part of its assessment.
Skeletal muscle vs muscle mass vs lean body mass
Three terms get used as if they mean the same thing, and they do not. This is where most confusion starts, usually while someone is staring at a body composition printout that lists lean body mass and skeletal muscle mass as two separate numbers and wondering why they differ.
| Term | What it includes | What an increase tells you |
|---|---|---|
| Skeletal muscle mass (SMM) | Skeletal muscle only, the trainable muscle attached to your bones | You genuinely gained muscle |
| Muscle mass (used loosely) | Often means all three muscle types: skeletal, smooth, and cardiac. In everyday use, people usually mean skeletal muscle. | Depends entirely on who is using the term |
| Lean body mass (LBM) | Everything that is not fat: muscle, body water, bone minerals, and organs | Could be muscle, could be water. Not conclusive on its own. |
How the three terms differ. Peer-reviewed work notes that skeletal muscle mass and lean body mass are frequently treated as synonyms when they measure different things.
The terms skeletal muscle mass and lean body mass are sometimes used as synonyms when they are not. SMM refers to the total weight of all skeletal muscles; LBM refers to the total weight of all non-fat components of the body.
— Frontiers in Nutrition, 2024
The practical consequence matters more than the semantics. Because body water makes up a large share of lean body mass, your LBM can climb after a salty meal or a hard training session without a single gram of new muscle. Skeletal muscle mass does not move that way. Every increase in skeletal muscle mass raises lean body mass, but the reverse is not true, which is why SMM is the number to watch if muscle is the goal.
What's a normal skeletal muscle percentage?
The most cited reference comes from a study that used whole-body MRI, the most accurate way to measure muscle, in 468 men and women aged 18 to 88.
Overall, skeletal muscle averaged 38.4 percent of body weight in men and 30.6 percent in women. The table below breaks that down by age group, showing skeletal muscle as a percentage of body weight (relative SM) alongside the absolute amount in kilograms.
| Age | Men, relative SM % | Men, total SM (kg) | Women, relative SM % | Women, total SM (kg) |
|---|---|---|---|---|
| 18–29 | 42.3 | 33.7 | 34.1 | 21.8 |
| 30–39 | 39.1 | 34.0 | 30.6 | 21.6 |
| 40–49 | 37.1 | 33.5 | 29.2 | 21.4 |
| 50–59 | 35.1 | 31.4 | 29.1 | 20.9 |
| 60–69 | 33.8 | 30.2 | 27.3 | 18.4 |
| 70+ | 36.0 | 27.8 | 30.2 | 18.0 |
| All adults | 38.4 | 33.0 | 30.6 | 21.0 |
Skeletal muscle mass by age and sex, measured by whole-body MRI. "Relative SM" is muscle as a percentage of body weight. Source: Janssen et al., Journal of Applied Physiology, 2000 (n = 468).
There is no single universal ideal skeletal muscle percentage, and anyone quoting one precise number as a target is overstating what the data supports. These are population averages, and the study found height and weight together explained about half the variation between people, so a taller, heavier person is not automatically better composed than a smaller one.
One row looks like a mistake but is not. The relative percentage ticks back up at 70-plus, to 36 percent in men and 30 percent in women, even though total muscle in kilograms keeps falling. That happens because relative SM is muscle as a share of body weight, and the oldest group also weighs less, which lifts the percentage. It is a good reminder that the percentage alone can mislead, and that the absolute kilograms and the trend over time matter alongside it.
Why your number changes between devices
If you have been measured on two different machines and got two different answers, nothing went wrong. No method weighs your muscle directly. MRI images it, DEXA infers it from X-ray attenuation, and bioelectrical impedance estimates it from how a small current passes through your body. Each applies a different model.
| Method | How it works | Typical setting |
|---|---|---|
| MRI | Images muscle tissue directly; the research reference standard | Research studies |
| DEXA | Low-dose X-ray; reports lean mass, fat, and bone | Clinics and imaging centers |
| Bioelectrical impedance (InBody) | Small current estimates water, then muscle and fat from it | Labs, gyms, some scales |
How the common methods differ. A 2024 analysis found a lack of agreement between methods and formulas for estimating skeletal muscle mass.
The rule that follows is simple: pick one method and stay with it. A skeletal muscle mass figure from a home scale, an InBody, and a DEXA are three different estimates of the same thing, and comparing them tells you nothing useful. Measuring the same way on the same device, and watching the direction over months, tells you a great deal. A body composition test is the practical way to establish that baseline, and Different Health captures it in-lab with InBody, which reports skeletal muscle mass and lean body mass as separate lines, plus a segmental breakdown of lean mass by arm, leg, and trunk, so the comparison stays consistent every time you retest.
What happens with age
Muscle is not static. Cleveland Clinic notes that adults typically lose about 3 to 5 percent of muscle each decade after age 30, with inactive people losing the most, and the loss becoming more noticeable and speeding up around 60.
The MRI data adds a useful wrinkle. Relative muscle, meaning muscle as a share of body weight, starts falling in the third decade. But a noticeable decline in absolute muscle mass did not show up until the end of the fifth decade, and it came mostly from the lower body. In other words, the percentage often drops first because body fat is climbing, not because muscle has gone anywhere yet.
There is a second nuance worth knowing. A quantitative review of the research found that strength is lost two to five times faster than mass, and that losing strength is a more consistent risk for disability and death than losing muscle mass. That is why grip strength is measured alongside body composition rather than instead of it. This is educational information rather than personal medical advice.
Why the number is worth tracking
Beyond movement, skeletal muscle is the body's largest insulin-sensitive tissue and accounts for the large majority of insulin-stimulated glucose clearance, which makes it central to blood sugar regulation. It also acts as a protein reserve the body draws on during illness or injury. Research in older adults links higher relative muscle mass with lower all-cause mortality, with body composition predicting survival better than body mass index.
Muscle does not extend life on its own. It matters because it captures strength, metabolic health, and physical resilience in one measurement. Building and keeping it comes down to progressive resistance training two to three times a week, enough protein spread across the day, and recovery, with gains arriving over months rather than weeks. Because muscle changes slowly, the trend is what to watch: Different Health tracks your skeletal muscle mass across repeat tests on the same InBody, so you can see the direction over time rather than reading a single number in isolation.
Key takeaways
- Skeletal muscle is the trainable muscle: the voluntary muscle on your bones, distinct from the smooth muscle in organs and cardiac muscle in your heart.
- Lean body mass isn't muscle: it's everything that isn't fat, including water, bone, and organs, so it can rise without any muscle gain.
- SMM is the honest number: every increase in skeletal muscle mass is real muscle; an increase in lean body mass might just be water.
- Reference averages: MRI data puts skeletal muscle at 38.4% of body weight in men and 30.6% in women, though there's no universal target.
- Devices don't agree: InBody, DEXA, and MRI estimates aren't interchangeable, so measure the same way and track the trend.
- Strength falls faster than mass: two to five times faster, and strength loss predicts disability and death more consistently.
Frequently asked questions
Is skeletal muscle mass the same as muscle mass?
Not exactly. Skeletal muscle mass is one of three muscle types: the voluntary muscle attached to your bones. "Muscle mass" used loosely can mean all muscle in the body, including the smooth muscle in your organs and the cardiac muscle in your heart, neither of which you can train. On a body composition readout, the number labeled skeletal muscle mass is the trainable muscle, which is why it is the figure worth tracking.
What does skeletal muscle mass mean?
Skeletal muscle mass is the total weight of all the skeletal muscles in your body, the voluntary muscle attached to your skeleton that moves you. It is measured in pounds or kilograms and often expressed as a percentage of body weight. It is the muscle that responds to resistance training, and it is distinct from lean body mass, which also includes water, bone, and organs.
What is a good skeletal muscle percentage?
In the reference study most often cited, whole-body MRI in 468 adults aged 18 to 88 found skeletal muscle averaged 38.4 percent of body weight in men and 30.6 percent in women. There is no single universal target, because the figure varies by age, sex, and measurement method. A percentage from one device should not be compared against a number from another; your own trend over time is the more useful measure.
What is the difference between skeletal muscle mass and lean body mass?
Skeletal muscle mass is muscle only. Lean body mass is everything in your body that is not fat, which includes muscle plus body water, bone minerals, and organs. They are frequently used as synonyms, and peer-reviewed work notes they should not be. Lean body mass can rise simply because you are holding more water, while a rise in skeletal muscle mass genuinely means more muscle.
Why is my skeletal muscle mass different on different devices?
Because no method measures muscle directly; each estimates it with a different model. A 2024 analysis comparing DXA, bioelectrical impedance, and anthropometry found a lack of agreement between methods and formulas for estimating skeletal muscle mass. That means an InBody figure and a DEXA figure are not interchangeable. Measure the same way each time and read the trend rather than the absolute value.
Does skeletal muscle mass decline with age?
Yes, though the shape is subtler than most summaries suggest. Cleveland Clinic notes adults typically lose about 3 to 5 percent of muscle per decade after 30, with loss becoming more noticeable and speeding up around 60. The MRI reference data found relative muscle as a percentage of body weight starts falling in the third decade, while a noticeable drop in absolute muscle mass did not appear until the end of the fifth decade.
References
- Janssen I, Heymsfield SB, Wang Z, Ross R. "Skeletal muscle mass and distribution in 468 men and women aged 18–88 yr." Journal of Applied Physiology. 2000;89(1):81–88.
- Cesaretti-Sansaloni P, et al. "Assessing skeletal muscle mass and lean body mass: an analysis of the agreement among dual X-ray absorptiometry, anthropometry, and bioelectrical impedance." Frontiers in Nutrition. 2024;11:1445892.
- Mitchell WK, Williams J, Atherton P, et al. "Sarcopenia, dynapenia, and the impact of advancing age on human skeletal muscle size and strength; a quantitative review." Frontiers in Physiology. 2012;3:260.
- Cleveland Clinic. "Sarcopenia (Muscle Loss): Symptoms & Causes."
- Srikanthan P, Karlamangla AS. "Muscle mass index as a predictor of longevity in older adults." The American Journal of Medicine. 2014;127(6):547–553.
- Wolfe RR. "The underappreciated role of muscle in health and disease." The American Journal of Clinical Nutrition. 2006;84(3):475–482.