What Is Skeletal Muscle Mass? Definition & Measurement

Skeletal muscle mass is the amount of muscle in your body that’s attached to your bones (your “move-your-body” muscle). If you’re asking what is skeletal muscle mass, that’s the core idea.

Skeletal muscle is “voluntary” muscle—meaning you control it on purpose—unlike cardiac muscle (heart) and smooth muscle (organs and blood vessels) (Cleveland Clinic).

You can measure skeletal muscle in a few common ways: MRI, DXA (dual-energy X-ray absorptiometry) body composition scans, and BIA (bioelectrical impedance analysis) smart scales.

Important nuance: many tools don’t directly “see” muscle tissue. They estimate proxies (like lean mass or fat-free mass) and then calculate a “muscle” number from that (Indian J Med Res, 2018).

If you’re trying to get leaner, stronger, or healthier as you age, skeletal muscle mass matters because it’s one of the most trainable and modifiable components of your body composition—and it’s closely related to strength and physical function (National Institute on Aging).

Important: This article is for general educational purposes and is not medical advice. Talk with a qualified clinician before making significant changes to your diet, supplements, or exercise routine—especially if you’re pregnant, managing chronic conditions, or recovering from an injury.

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Pairing DEXA with Skeletal Muscle Mass

Learn how BodySpec DEXA scans can help if you want to learn about your skeletal muscle mass.

Book a BodySpec DEXA scan today and see exactly how your body composition changes over time.

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Quick definitions (for clearer muscle metrics)

You’ll often see multiple “muscle” metrics online. Here’s a simple way to keep them straight:

• Skeletal muscle: Voluntary muscle attached to bones (it’s also “striated,” meaning it looks striped under a microscope).
• Skeletal muscle mass (SMM): A label used by some devices and reports for “muscle.” The definition and calculation can vary by method and manufacturer. For example, BIA systems can differ in electrode setups, measurement frequencies, and proprietary algorithms—so numbers may not be interchangeable from device to device (Curr Opin Clin Nutr Metab Care, 2024).
• Lean mass (lean soft tissue): What’s left after you subtract fat and bone on a DXA report. It’s muscle plus other soft tissues and fluids—so it’s muscle-relevant, but not “pure muscle.”
• Lean body mass (LBM) and fat-free mass (FFM): In practice these are often used interchangeably, but definitions vary historically. A critical review argues that LBM (as originally defined) is chemically equivalent to FFM and recommends using FFM for clarity (Adv Nutr, 2024).

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What counts as skeletal muscle (and what doesn’t)?

Includes:

• Muscles that move your skeleton (quads, glutes, lats, pecs, biceps, core, etc.)

Doesn’t include:

• Cardiac muscle (your heart)
• Smooth muscle (organs like intestines, uterus, and blood vessels)

This distinction matters because some sources use the term “muscle mass” to encompass all muscle types (including organs). However, in fitness, sports, and sarcopenia contexts, skeletal muscle is the priority focus. Major education resources distinguish the three muscle types by structure (striated vs. smooth) and control (voluntary vs. involuntary) (NIH SEER Training; MedlinePlus).

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What does skeletal muscle do for your body?

Skeletal muscle is more than “the stuff that makes you look toned.” It’s a multi-purpose system that supports day-to-day function.

Functions commonly highlighted in clinical education include:

• Moving and stabilizing your skeleton and joints
• Maintaining posture
• Supporting breathing (muscles that expand/contract the chest)
• Chewing and swallowing
• Storing nutrients and helping regulate body temperature

These functions are summarized in clinical educational resources on muscle tissue (Cleveland Clinic).

At the tissue level, muscle size changes over time because muscle protein is constantly being built and broken down (protein turnover). A review discusses how protein turnover—including both synthesis and degradation—helps regulate skeletal muscle mass, and how exercise can influence these processes (AJP-Cell Physiol, 2021).

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Why skeletal muscle mass matters (even if your goal isn’t bodybuilding)

1) It’s tied to strength, mobility, and independence with age

Age-related muscle loss (sarcopenia) is one reason everyday tasks can start feeling harder over time.

If you want the deep dive: Sarcopenia: The Complete Guide to Age-Related Muscle Loss.

2) It changes how your body composition progress actually looks

If you lose 5 pounds of fat and gain 5 pounds of muscle, the scale says “no change”—but your body composition and performance picture may be very different.

That’s why body composition tracking matters: it helps you separate changes in fat from changes in lean tissue.

3) It’s one of the most trainable “health assets” you own

You can’t control everything about aging, but you can meaningfully influence skeletal muscle with:

• Progressive resistance training (gradually increasing reps, sets, and/or weight over time)
• Adequate protein
• Sufficient recovery

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How much skeletal muscle mass is “normal”? (benchmarks + reality check)

First, an important caveat: there isn’t one universal “ideal” skeletal muscle mass number because it changes with height, sex, genetics, sport demands, and body fat level.

A major review on skeletal muscle reference values emphasizes that comparisons only make sense when you’re using the same measurement method/device and an appropriate reference population (Nutrients, 2020).

At-a-glance benchmarks you’ll see most often (including skeletal muscle mass percentage)

Quick interpretation note: The examples below come from specific studies and guideline papers (different ages, countries, and devices). They’re useful for context, but they’re not universal targets.

Why the ranges vary: “Low muscle mass” cutoffs depend on the guideline framework and reference population (for example, EWGSOP2 = European Working Group on Sarcopenia in Older People, 2nd consensus) and may also vary by ethnicity, scanner brand, and the specific index used (Age and Ageing, 2019).

A more practical way to benchmark in real life (DXA-derived ALM and ALMI)

Here’s the plain-English issue: most people don’t get an MRI to “measure muscle,” and most at-home devices estimate muscle indirectly.

So in clinics and research, it’s common to use a DXA report’s limb lean mass as a muscle-relevant stand-in. If you’ve ever seen a DXA report that lists lean mass for each arm and leg (or a metric like ALM/height²), that’s what feeds into the two metrics below.

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ALM vs. ALMI: what they are (and why ALMI is used more often)

You’ll see both spellings online: DXA and DEXA. They refer to the same technology (dual-energy X-ray absorptiometry). In this article we’ll use DXA for consistency.

A DXA body composition scan provides an analysis of fat, lean mass, and bone and typically generates a report with multiple metrics you can track over time (UC Davis Health).

ALM (appendicular lean mass)

• ALM = lean mass of your arms + legs (reported in kg)

Why it’s used: DXA-measured appendicular lean mass (ALM) is widely used as a practical proxy for skeletal muscle mass in sarcopenia research and clinical practice, even though it isn’t a perfect one-to-one measure of “pure muscle” (Age and Ageing, 2019; Clin Nutr ESPEN, 2022).

ALMI (appendicular lean mass index)

• ALMI = ALM (kg) ÷ height² (m²)

Why ALMI matters:

• It normalizes limb lean mass for body size, making comparisons more meaningful across different heights.
• It’s used frequently in sarcopenia research and clinical screening as a standardized way to describe “how much limb lean mass you have for your height.”

Practical interpretation: ALM tells you how much limb lean mass you have. ALMI helps answer whether that amount is “a lot” or “a little” for your height.

If you have a DXA report and want help translating “lean mass” metrics into something muscle-relevant, start here: How to Measure Muscle Mass (At-Home vs. Clinical Methods).

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How to measure skeletal muscle mass (from most accurate to most accessible)

1) MRI (very accurate, not convenient)

Whole-body MRI can directly quantify skeletal muscle volume and mass and is often used in research settings.

Downsides: cost and access.

2) DXA body composition scan (highly practical for tracking trends)

A DXA scan separates body composition into fat mass, lean mass, and bone mineral content, and provides regional lean mass (arms, legs, trunk). That regional detail is one reason DXA is widely used for body composition assessment (UC Davis Health).

Clinical context note: DXA-derived ALM/ALMI can support screening and monitoring, but diagnosis and interpretation should be done by a qualified clinician using the full clinical picture (not a single number in isolation).

Important nuance: DXA measures lean mass, not pure muscle. And like any method, it can be influenced by acute shifts in hydration and glycogen (stored carbohydrate in muscle) status—making consistent prep important for clean trend tracking (Med Sci Sports Exerc, 2017; Radiol Bras, 2022).

Learn the tech in plain English: How a DXA scan works.

Because DXA provides regional lean mass (including arms and legs), it’s a useful baseline for tracking ALM/ALMI over time. Want a baseline you can repeat? Book a BodySpec scan.

3) Bioelectrical impedance (BIA) scales and “muscle mass” estimates (convenient, variable)

BIA devices estimate fat-free mass from electrical conductivity and then use equations to report “skeletal muscle mass.”

A 2024 study in healthy young adults compared DXA (reference) with BIA and anthropometry. (Anthropometry means body measurements like circumferences and skinfolds.) It found poor to moderate concordance across most formulas and methods, meaning the results often weren’t interchangeable (Frontiers in Nutrition, 2024).

In plain terms: muscle numbers can change a lot when you switch devices or formulas—even if your body hasn’t changed.

If you use BIA:

• Use the same device.
• Test under the same conditions (time of day, hydration, food, exercise).
• Treat it as a trend tool, not an absolute truth.

If you want to understand why smart scales swing so much (and how to use them anyway), see: How Accurate Are Smart Scales?.

4) Tape measure and circumference methods (lowest cost, lowest precision)

Circumference measures (like upper arm, thigh, or calf size) can be useful for tracking local changes when you’re consistent, but they can’t tell you how much of the change is fat vs. muscle.

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How to increase skeletal muscle mass (evidence-based playbook)

1) Strength train at least 2 days per week (then build from there)

For general health, the U.S. Physical Activity Guidelines for Americans recommend muscle-strengthening activities of moderate or greater intensity involving all major muscle groups on 2 or more days a week (Physical Activity Guidelines, 2018; health.gov guideline hub).

If you’re aiming to build muscle, many people progress beyond that minimum—gradually increasing training volume and loads over time.

If you’re new and want a simple plan you can actually repeat: Strength Training for Beginners.

If you’re older or want joint-friendly options: Weight Training After 60.

2) Eat enough protein (especially during fat loss)

If you’re trying to add muscle—or protect it while dieting—protein is a major lever.

Most protein-intake studies are in healthy adults doing resistance training. Needs vary with age, goals, and total calories—and people with kidney disease or other conditions may need individualized targets from a clinician.

Here are two evidence-based anchors:

• A systematic review and meta-analysis found that protein supplementation increases fat-free mass and strength gains during resistance training, and suggests benefits plateau above total daily intakes around ~1.62 g/kg/day for fat-free mass in many adults (Br J Sports Med, 2018).
• A newer systematic review/meta-analysis found that increasing daily protein intake leads to small additional gains in lean body mass when combined with resistance exercise, with effects depending on age and protein dose (e.g., ≥1.6 g/kg/day in younger adults; ~1.2–1.59 g/kg/day in older adults in their subgroup analyses) (Nutrients, 2022).

A simple starting point (with more nuance in our guide): How Much Protein to Prevent Muscle Loss?.

3) Don’t ignore recovery

Muscle growth is the adaptation after training. When sleep is consistently short, strength and performance can suffer in many people—making it harder to train well and recover well (Sleep Breath, 2025).

4) Track what matters (and retest at a sensible interval)

In the real world, it’s hard to “feel” a small muscle gain. A measurement system helps you confirm whether you’re actually building/preserving lean tissue.

A simple tracking stack:

• Weekly (optional): body weight under consistent conditions
• Monthly to quarterly: a body composition checkpoint using the same method and similar prep

Timing note: There’s no single perfect retest interval. As a practical (non-clinical) rule of thumb, many people choose something like 8–12 weeks during a training block to allow enough time for measurable change—especially if they’re using a method like DXA that’s designed for repeatable tracking.

As an illustrative example (not a universal timeline), one study in women found thigh muscle thickness increased early in an 8-week resistance training program and exceeded measurement error after ~2–3 weeks in several sites (Geriatr Gerontol Int, 2017).

If you’re using DXA, standardizing your prep improves apples-to-apples comparisons: Prepare for Your BodySpec Scan.

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FAQ: “what is skeletal muscle mass” questions people also ask

Does skeletal muscle mass include body water?

Not as a concept—skeletal muscle is the tissue itself.

But measurements can be influenced by water:

• BIA-based “muscle mass” estimates can swing noticeably with hydration, sodium, food intake, and recent exercise (because they’re built from body-water assumptions) (Indian J Med Res, 2018).
• DXA “lean mass” can also shift a bit with changes in fluids and glycogen, so consistent prep matters for clean comparisons (Med Sci Sports Exerc, 2017).

For practical prep, see: Prepare for Your BodySpec Scan. For at-home trend tracking, see: How Accurate Are Smart Scales?.

Is skeletal muscle mass the same as lean body mass?

No. Lean body mass includes skeletal muscle plus other non-fat tissues. Skeletal muscle mass is just the muscle attached to your skeleton.

What’s the best test for skeletal muscle mass?

For direct anatomical muscle measurement, advanced imaging like MRI is excellent but not convenient. For most people who want a reliable, repeatable body composition checkpoint, DXA is a practical option because it provides regional lean mass plus ALM/ALMI in minutes.

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The bottom line

Skeletal muscle mass is the amount of voluntary, bone-attached muscle you carry—and it’s one of the most important (and modifiable) components of body composition.

If you want to take action, the simplest path is:

1. Establish a baseline (ideally with a method that provides regional lean mass)
2. Strength train consistently (at least 2×/week, then progress)
3. Hit a protein target you can sustain
4. Re-check your numbers every couple of months using the same method and similar prep

Quick safety note: Training and nutrition targets should be individualized. If you have medical conditions (or you’re older, pregnant, or recovering from injury), consider checking in with a clinician and/or registered dietitian before making big changes.

If you want a deeper comparison of measurement options (including DXA vs. BIA), see: How to Measure Muscle Mass.