SMM
Skeletal Muscle Mass (SMM)
Skeletal Muscle Mass is an estimate of the amount of voluntarily controlled muscle in kilograms — the muscle between bones and joints that produces movement.
Short answer
Skeletal muscle mass is an estimate of voluntarily controlled muscle and it changes over months, not days. The manufacturer publishes neither the equation nor the relationship to soft lean mass, so treat it as a trackable trend under identical conditions rather than a precisely verifiable number.
- Skeletal muscle mass30.4 kg
- Rest of body weight36.6 kg
What the value represents
Muscle in the body comes in three kinds: cardiac, smooth and skeletal. The result sheet follows skeletal muscle exclusively — the muscle lying between bones and joints that you control voluntarily. According to the manual it is precisely this muscle that changes readily with physical activity.
The value is given in kilograms and is one of the three bars in the Muscle-Fat Analysis, alongside weight and fat mass. Bar length is relative to the ideal weight derived from your height. The manual adds an interpretive rule: skeletal muscle mass at 100 percent means a sufficient amount of muscle relative to ideal weight.
It is only fair to say how the value comes about: it is calculated, but the manufacturer manual gives neither an equation for it nor its relationship to soft lean mass. The table of contents promises a chapter on the relationship between muscle mass and soft lean mass, but the text does not cover it. The value therefore cannot be independently recalculated or checked against the other numbers on the sheet.
- The estimated amount of skeletal muscle in kilograms.
- The muscle responsible for voluntary movement — the muscle training directly affects.
- It does not represent cardiac or smooth muscle.
- It does not represent muscle strength, endurance or function. Amount and function are related but not the same thing.
How the value is produced
- A calculated value. The manufacturer manual gives no equation for the calculation — that is a documented gap in the documentation, not a shortcoming of this text.
- The manual likewise gives no relationship between skeletal muscle mass and soft lean mass, even though its table of contents promises such a chapter. The two values therefore cannot be converted into one another.
- The starting input for the whole chain is body water derived from impedance. Muscle is the main water reservoir in the body, so the muscle estimate stands or falls with the water estimate.
- The manual notes that trunk muscle makes up almost half of total muscle mass. This is why the trunk is measured separately — even a small change in its impedance has a large effect on the whole-body result.
- The manufacturer internal validation study against DEXA covered fat free mass, fat mass and segmental lean mass. Skeletal muscle mass is not among the validated parameters.
How to read the current value
- Read the value against the reference range in brackets and against the bar length in the Muscle-Fat Analysis. The manual states that 100 percent means a sufficient amount of muscle relative to ideal weight — not relative to your current weight.
- The manual gives no numeric boundaries for the Under, Normal and Over bands in the Muscle-Fat Analysis. Work with the bar position and the reference range rather than inventing thresholds.
- Absolute kilograms cannot be compared between people. The Skeletal Muscle Index (SMI), which relates the value to height, serves comparison with an age group.
- If the segmental ECW/TBW ratio is raised while muscle looks high, treat the value from that measurement with more caution. The manual explicitly warns about overstatement through retained fluid.
How to read change over time
- Muscle mass changes slowly. A meaningful horizon for tracking is several months, not weeks — and certainly not days.
- Short-term swings come mainly from hydration, because muscle is the main water reservoir and the estimate is derived from water.
- During weight reduction the key question is whether muscle mass holds steady. Weight falling with muscle preserved is a different situation from both falling.
- A rise in muscle mass together with a rise in protein, intracellular water and body cell mass over months is a consistent picture of a real gain in tissue.
General principles: how to read change between measurements →
What to read it with
- Skeletal Muscle Index (SMI) – normalisation to height for comparison with an age group.
- Skeletal muscle mass to weight – a complementary view of the same value from the weight side.
- Fat mass and weight – only the three bars of the Muscle-Fat Analysis together give the picture.
- Segmental lean mass – shows whether muscle is distributed evenly between limbs and trunk.
- The segmental ECW/TBW ratio – a check on whether the value has been overstated by retained fluid.
- Hand grip strength – a functional measure that adds information the amount alone does not carry.
- Muscle control – the recommended adjustment derived from this very value.
Common misunderstandings
- Skeletal muscle mass is not fat free mass. Fat free mass also includes water, bone and organs.
- Nor is it soft lean mass. The manual separates the two explicitly and gives no relationship between them.
- More muscle mass does not automatically mean more strength. Expert criteria for assessing muscle always combine amount with a measurement of strength.
- Low muscle mass is not a diagnosis. The revised European consensus EWGSOP2 puts muscle strength first and uses low muscle quantity only as a confirming criterion.
- Values are not comparable across devices from different manufacturers or across measurement methods.
What can affect the result short term
- Hydration status, because the estimate starts from body water and muscle is its main reservoir.
- Training shortly before the measurement, which changes perfusion and fluid distribution. The manual recommends not exercising beforehand.
- Sauna and hot or cold showers, which change circulation according to the manual.
- Food and drink before the measurement; the manual recommends measuring fasted.
- Time of day — according to the manual water shifts towards the lower half of the body during the day.
- Body position before the measurement; the manual recommends standing for at least 5 minutes.
- The quality of electrode contact and correct posture: arms straight and not touching the trunk, thighs not touching each other.
What to do next
The explanation of the value is the same for everyone. Only the suggested step changes — pick whichever fits you.
- Follow muscle mass at intervals of months and always under comparable conditions. A shorter horizon shows mainly water fluctuation.
- During weight reduction use muscle mass as a control value: the aim is for fat mass to fall while muscle holds.
- For comparison with an age group use the muscle index, not absolute kilograms.
- If the functional state of your muscles is what matters, add a grip strength measurement. Amount alone does not say how well muscle works.
- When the ECW/TBW ratio is raised, confirm the value at another measurement under better conditions before drawing conclusions from it.
What the value does not determine
- Neither the calculation equation nor the relationship to soft lean mass is published by the manufacturer.
- It is not among the parameters validated against DEXA in the manufacturer internal study.
- It does not measure muscle strength or function.
- It does not diagnose sarcopenia or any other condition.
- It is sensitive to hydration status and can be overstated where fluid is retained.
- Values are not interchangeable between devices or between measurement methods.
On terminology
- The manufacturer distinguishes skeletal muscle mass (SMM), soft lean mass (SLM) and fat free mass (FFM). They are not synonyms, and the manual gives no conversion between SMM and SLM.
- The 100 percent mark in the Muscle-Fat Analysis relates to ideal weight according to the manual, not to current weight.
When to discuss it with a professional
If you notice a sustained loss of muscle mass together with weakness, difficulty walking or fatigue, that belongs with a doctor or physiotherapist. The measurement itself cannot establish the cause.
In more depth, with sourcesIn more depth: what the manufacturer does not state about the muscle calculationExpand the detailed explanation
The manual classes skeletal muscle mass as a calculated value but gives no equation for it. It also gives no relationship to soft lean mass, even though the table of contents announces such a chapter — the text then describes soft lean mass only. In practical terms this means the value cannot be derived or checked from the other numbers on the sheet, and nothing more can be claimed about its construction than what the manufacturer itself has published.
What can be documented is the input chain. Bioimpedance measures resistance and reactance in five segments at eight frequencies, body water is derived from that, and the remaining components from body water. On segmental measurement the manual notes that trunk muscle makes up almost half of total muscle mass, which is why the trunk is measured separately: its short and wide geometry gives it low impedance, so even a small deviation in its measurement propagates markedly into the whole-body result. The manufacturer internal validation study against DEXA included segmental lean mass with correlations of r ≥ 0.95, but did not validate skeletal muscle mass as a separate parameter.
For expert use it is therefore essential to separate the amount of muscle from its function. The revised European consensus EWGSOP2 puts muscle strength first in the assessment pathway and uses low muscle quantity only as a confirming criterion. The ESPEN clinical guidelines on bioimpedance add a warning about the sensitivity of estimates to altered fluid distribution — which affects muscle mass directly, because muscle holds most of the intracellular water the estimate is derived from.
Sources
- InBody 970 Result Sheet Interpretation (ENG_A_211104) — InBody Co., Ltd.
- Validation of InBody 970 (BWA) against DEXA — InBody Co., Ltd.
- Bioelectrical impedance analysis – part II: utilization in clinical practice — Clinical Nutrition (ESPEN guidelines)
- Sarcopenia: revised European consensus on definition and diagnosis (EWGSOP2) — Age and Ageing
On this page
Related values
Commonly confused with
Related articles
Where to find it
The first table at the top of the result sheet, labelled Body Composition Analysis.
- Reviewed by
- MojeInBody content team
- Date
An InBody measurement is not a diagnostic tool. Results do not determine health status and do not replace a medical examination.
FAQ
Frequently asked questions
Answers to what people ask most often about this value.
How does InBody actually calculate muscle mass?
The manufacturer manual does not give the equation. It describes the value as calculated and states that the whole model starts from body water derived from impedance, but does not publish the specific relationship. It also gives no relationship to soft lean mass. The value therefore cannot be independently verified.
How long before muscle mass measures differently once I start training?
Count in months rather than weeks. The manual recommends no particular interval between measurements, but muscle tissue grows slowly and short-term differences on the sheet mainly reflect fluctuation in body water.
My muscle mass is low. Does that mean sarcopenia?
No. Under the revised European consensus EWGSOP2 the assessment starts from a measurement of muscle strength, and low muscle quantity serves only as a confirming criterion. A bioimpedance value on its own does not establish a diagnosis.
Why did my muscle mass fall even though I trained?
Most often because of the measurement conditions. The muscle estimate starts from body water, so dehydration after training, a different time of day or different food beforehand will shift it. Always compare measurements taken under comparable conditions and months apart.
Explanations make the most sense alongside your own trend.
The public guide is open to everyone. The client portal adds your own value, history and the context of a specific measurement.