Skeletal muscle mass (SMM)

The mass of skeletal muscle

Skeletal muscle mass — the muscle fibers of the body

Skeletal muscle mass (SMM) is the muscle we move the body with. It makes up about half of fat-free mass and up to 30–40% of body weight.

Muscle is not only about strength. It is the main consumer of glucose and an "organ of longevity": the more and the more active the muscle mass, the better the metabolism and resistance to disease. Its age-related loss is called sarcopenia.

Reference ranges

It is assessed through the skeletal muscle index (SMI = SMM / height²) and the share of weight:

Share of muscle in body weight	≈ 30 – 40%
Share of muscle in fat-free mass	≈ 50%
Assessment	by the SMI index (muscle / height²)
Sarcopenia	low SMI + reduced strength

Important: Threshold values depend on sex, age and method. The key criterion of sarcopenia is not only mass but also a decline in muscle strength and function.

What an elevated level means

High skeletal muscle mass is a favorable sign: greater strength, higher energy expenditure at rest, better blood-sugar control and insulin sensitivity, stronger bones.

Building muscle is beneficial at any age — it is a proven way to improve metabolism and reduce the risks associated with aging.

What a low level means

Low muscle mass reduces strength and metabolism and increases vulnerability. Age-related muscle loss — sarcopenia — leads to weakness, falls, fractures and a loss of independence.

Especially dangerous is the combination of low muscle and high fat mass (sarcopenic obesity): the weight may be normal, but the functional state is poor.

How the test is done

Skeletal muscle mass is assessed by bioimpedance or more accurately DEXA; from it the SMI index is calculated (muscle divided by height squared). Muscle function is additionally checked by strength tests (for example, grip strength) and walking speed.

The trend matters: the goal is to preserve and build muscle, especially with age.

What affects the value

Strength training — the main way to build muscle.
Enough protein — necessary for muscle synthesis.
Age — muscle is lost at about 1% a year after 40–50.
Immobility and bed rest — quickly reduce mass.
Hormones (testosterone, IGF-1) — affect muscle.
Hydration — affects the bioimpedance estimate.

Assess body composition

Assess your muscle and fat-free mass within your body composition:

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

Frequently asked questions

How much muscle should the body have?

Skeletal muscle makes up about 30–40% of body weight and about half of fat-free mass, but the exact norms depend on sex, age and method. What matters more than an absolute figure is the muscle index (SMI) and whether the mass is preserved with age.

Why is muscle called an “organ of longevity”?

Muscle is the main consumer of glucose and a source of beneficial signaling molecules (myokines). Adequate muscle mass improves metabolism and insulin sensitivity and protects against the consequences of aging, so preserving it is an important goal of healthy longevity.

Can muscle be built in old age?

Yes. Muscle responds to strength exercise at any age. Combining regular strength training with enough protein makes it possible to slow and partly reverse age-related muscle loss (sarcopenia).

Sources

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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