SMI
Skeletal Muscle Index (SMI)
Skeletal Muscle Index relates limb muscle mass to height squared so it can be compared between people of different build.
Short answer
The index converts muscle mass onto a height basis so it can be compared with your age group. Follow it across months, ideally next to grip strength, because the amount of muscle does not tell you how well it works.
What the value represents
Muscle mass in kilograms compares badly between people, because a taller person naturally carries more muscle. The index removes that effect by dividing muscle mass by height squared — the same normalisation BMI applies to weight.
The result is expressed in kg/m² and is meant for comparison against an age group, not for judging strength or performance.
The index says nothing about how well muscles work. Muscle mass and muscle strength are related but they are not the same thing.
- Muscle mass related to body height, in kg/m².
- An indicator used to assess the amount of muscle independently of build.
- It does not represent muscle strength, endurance or functional capacity.
How the value is produced
- A calculated value from the muscle mass estimated by bioimpedance and the entered height. The research parameter list defines it as appendicular lean mass divided by height squared.
- On the Evaluation Result Sheet the manual presents it as a chart of the mean and standard deviation by age across the range of 20 to 80 years, built from manufacturer data.
- The manual marks a cut-off line on that chart below which the state is labelled as under, but does not print the numeric value of the cut-off itself.
How to read the current value
- Read the value against your age group rather than one universal threshold. Muscle mass declines naturally with age and the manufacturer chart accounts for that.
- Sitting inside the shaded band of the chart means you are within the mean and standard deviation for your age.
- A value below the cut-off line is a prompt for attention according to the manufacturer, not a diagnosis. Expert criteria for low muscle mass always combine the amount of muscle with a measurement of strength.
- The only numeric SMI thresholds the manual prints are the Asian sarcopenia criteria of 7.0 kg/m² for men and 5.7 kg/m² for women, given alongside the research parameter. The cut-off used on the evaluation chart is not stated numerically.
How to read change over time
- The index moves slowly. Differences between measurements weeks apart usually sit inside ordinary variation.
- A meaningful horizon is several months, especially with deliberate strength training or when watching for muscle loss.
- Because height does not change, the index effectively tracks the development of muscle mass on a stable denominator.
General principles: how to read change between measurements →
What to read it with
- Skeletal Muscle Mass in kilograms – the absolute value the index is built from.
- Hand grip strength – a functional measure that does not merely add to the amount of muscle but fundamentally qualifies it.
- Segmental lean mass – shows whether muscle is distributed evenly.
- Phase angle – another value commonly followed alongside muscle mass.
Common misunderstandings
- A low index is not a diagnosis of sarcopenia. The European consensus criteria require low muscle strength together with low muscle quantity, and strength is the entry point.
- A high index does not automatically mean high strength. Quantity and function can diverge.
- The index cannot be compared across measurement methods. A value from bioimpedance and one from another method are not interchangeable.
What can affect the result short term
- Hydration, because muscle mass is estimated from body water.
- Training shortly before the measurement and the shift of fluid over the course of the day.
- The accuracy of the entered height, which sits in the denominator.
What to do next
The explanation of the value is the same for everyone. Only the suggested step changes — pick whichever fits you.
- Follow the index together with grip strength if what you care about is the functional state of your muscles.
- Use a horizon of months rather than weeks when tracking muscle development.
- If the value stays low and is accompanied by weakness or falling performance, take it to a doctor or physiotherapist.
What the value does not determine
- It does not measure muscle strength or function.
- It does not diagnose sarcopenia or any other condition.
- Values are not interchangeable between measurement methods.
- It is sensitive to changes in hydration and to the accuracy of the entered height.
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. The measurement itself cannot establish the cause.
In more depth, with sourcesIn more depth: normalising to height and the link to expert criteriaExpand the detailed explanation
Normalising muscle mass to height squared is a standard step that makes people of different build comparable. It also ties the value to both the quality of the muscle mass estimate and the accuracy of the entered height, since both quantities sit in the same fraction.
The revised European consensus EWGSOP2 places muscle strength first in the assessment pathway and uses low muscle quantity only as a confirming criterion. An index from bioimpedance therefore neither confirms nor excludes any condition on its own; it is one input among several.
The ESPEN clinical guidelines on bioimpedance add that estimates of muscle mass are sensitive to altered fluid distribution. Where fluid retention is present, interpreting the index becomes harder and requires context the device does not have.
Sources
- InBody 970 Result Sheet Interpretation (ENG_A_211104) — InBody Co., Ltd.
- Sarcopenia: revised European consensus on definition and diagnosis (EWGSOP2) — Age and Ageing
- Bioelectrical impedance analysis – part II: utilization in clinical practice — Clinical Nutrition (ESPEN guidelines)
On this page
Related values
Related articles
Where to find it
The Muscle-Fat Analysis bar chart and the Obesity Analysis section that follows it.
- 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.
Does a low index mean I have sarcopenia?
No. The European consensus EWGSOP2 starts from a measurement of muscle strength and treats low muscle quantity as a supporting criterion. An index from bioimpedance on its own does not establish a diagnosis.
Why does my index barely move even though I train?
Muscle mass grows slowly, and the index additionally divides the value by height. Changes usually show up over months rather than weeks.
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.