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SMI — Skeletal Muscle Index

Muscle mass / height²

A cross-section of skeletal muscle with an index diagram

SMI (Skeletal Muscle Index) normalizes muscle mass to height so that people of different sizes can be compared. Most often it is calculated as appendicular muscle mass (ASM) divided by height squared.

SMI is a key measure for diagnosing sarcopenia, the age-related loss of muscle. It is closely tied to skeletal muscle mass and lean body mass, but, unlike FFMI, it accounts specifically for the muscle of the limbs.

Reference ranges

Thresholds for "low muscle mass" per EWGSOP2 (European consensus, measured by DXA):

ASM / height², men< 7.0 kg/m²
ASM / height², women< 5.5 kg/m²
Absolute ASM, men< 20 kg
Absolute ASM, women< 15 kg
Important: The thresholds depend on the method: for bioimpedance (BIA) and different ethnic groups the values differ. A low SMI supports but does not replace a diagnosis of sarcopenia (strength and function are also needed).

What an elevated level means

A high SMI means a good reserve of muscle mass, which is linked to better metabolism, strength and resistance to disease. There is no special "too high" threshold: in athletes high values are normal.

What a low level means

A low SMI (below the thresholds above) indicates a muscle deficit. Combined with reduced grip strength (men <27 kg, women <16 kg) and slow walking (≤0.8 m/s) it confirms sarcopenia and means a risk of falls, disability and increased mortality.

How the test is done

Appendicular muscle mass (ASM) is measured by DXA or bioimpedance (BIA), less often by MRI/CT. Then the index is calculated: SMI = ASM / height(m)². There is also the Janssen variant, where SMM is expressed as a percentage of body mass.

What affects the value

  • Measurement method (DXA, BIA, MRI) gives different absolute values and thresholds.
  • Hydration and edema distort bioimpedance results.
  • Ethnicity and age affect the reference values.

Calculate using this marker

Calculators where SMI (Skeletal Muscle Index) is used directly:

Body Composition Body fat, muscle mass (FFMI, SMM, SMI), body shape, and ABSI — all in one Open

Frequently asked questions

What SMI is considered normal?

Per EWGSOP2, low muscle mass is ASM/height² below 7.0 kg/m² in men and 5.5 kg/m² in women (DXA method). Values above these thresholds are normal; for bioimpedance the figures differ.

Is a low SMI already sarcopenia?

No, it is only one of the criteria. A diagnosis of sarcopenia also requires reduced muscle strength (for example, a weak grip) and/or impaired physical function, such as slow walking.

Related terms

Skeletal muscle mass (SMM) The mass of skeletal muscle Sarcopenia Age-related loss of muscle mass and strength Fat-free mass (FFM) The whole body except fat FFMI (Fat-Free Mass Index) Fat-free mass / height² Lean body mass Body mass without fat — muscle, bone, water, organs DEXA (Dual-energy X-ray absorptiometry) The reference method for assessing body composition

Sources

  1. Cruz-Jentoft et al., 2019 (EWGSOP2) — Sarcopenia: revised European consensus
  2. Wikipedia — Lean body mass
info
This content is educational and does not replace consultation with a physician. Reference ranges may vary depending on the laboratory and method — rely on the ranges stated on your own test report.
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