Muscle mass: how it is measured, the skeletal muscle index, and what impedance cannot see

What do people mean by muscle mass?

Muscle mass is not one number. Papers and devices use several names for related things: skeletal muscle mass, appendicular skeletal muscle mass (the muscle in the arms and legs), lean mass and fat-free mass. A review of reference values found they are not interchangeable. Studies differ in method and in how the mass is adjusted for body size, which leads to different criteria between studies and even between consensus definitions.²

The practical consequence is simple. A figure on your scale labeled “muscle mass” and a figure in a clinic report may be measuring different things. Before you compare, check which quantity each one gives.

This guide uses the vocabulary of the EWGSOP2 consensus, an expert statement on sarcopenia, a muscle disease more common in older adults but one that can occur earlier in life.¹ That consensus is written for case-finding in health care, not for fitness goals. It is still the clearest published place to see how muscle quantity is expressed and where its cutoffs fall.

The skeletal muscle index, with calculated examples

The EWGSOP2 consensus lists two ways to express low muscle quantity: appendicular skeletal muscle mass (ASM) in kilograms, or ASM divided by height squared. The authors say ASM can be reported in absolute terms or adjusted for body size.¹ Its cutoffs are in the table below.

To use the index you need ASM in kilograms and height in meters. If you only have pounds and inches, convert: pounds × 0.4536 for kilograms, inches × 0.0254 for meters. Then divide kilograms by meters squared.

A low index is not a verdict. The consensus puts low muscle strength first. It uses low muscle quantity or quality to confirm the case, and it treats poor physical performance as a sign of severe sarcopenia.¹ A number below the line is a reason to ask a clinician, not a conclusion.

Why strength is part of the picture

Because the consensus centers on strength, it also publishes cutoffs for simple tests. A grip strength below 27 kg for men or below 16 kg for women, or a chair stand taking longer than 15 seconds for five rises, counts as low strength under EWGSOP2.¹

A scale cannot give you these. Neither a smart scale nor a scan measures how hard you can grip or how quickly you can stand up. Muscle quantity and muscle function are different questions, which is why the guide to muscle loss after 50 and the strength training guide for seniors pair measurement with exercise.

How is muscle mass measured?

In a clinic, the EWGSOP2 lists DXA for appendicular muscle mass, with muscle mass predicted from impedance as another option.¹ A review of reference values adds that single CT or MRI images are used as well.²

A review of reference values explains the link to the whole body. Single CT or MRI slices and appendicular lean soft tissue from DXA or BIA are valid substitutes for total skeletal muscle, because they correlate highly with whole-body imaging. But random error limits their use for individual cases, and together with systematic error it limits the accurate detection of changes.² Results can only be compared with reference values obtained by the same method, the same device and a suitable reference population.²

What impedance scales can and cannot measure

A bioelectrical impedance scale sends a small current between electrodes and turns the result into estimates with built-in equations. A review of the method notes that its assumptions include a body made of cylinders, fat-free mass containing virtually all the body's water, and constant hydration. Equations differ by the sample they came from, and age and ethnicity could sensitively affect the estimates.³

Two studies compared consumer impedance devices with imaging:

• In a 12-month weight-loss study of 19 adults, impedance followed the loss of muscle well enough, but underestimated skeletal muscle by 1 to 2 kg at each time point compared with DXA. The authors called impedance a useful method for skeletal muscle.⁴
• In 200 healthy Chinese adults, two single-frequency devices correlated well with MRI and DXA for skeletal muscle percentage (r = 0.85 and 0.89), with small biases in most measures.⁵

Neither study says a scale can replace a scan. They say a scale tracks the broad direction. Meanwhile, the American Society for Parenteral and Enteral Nutrition looked at body composition methods in clinical populations. It found that the validity of DXA for lean mass remains unknown in any clinical population, and it made no recommendation for BIA because manufacturers keep their regression models proprietary.⁶

No institutional source read for this guide gives a healthy “muscle mass percentage” range for the general public, so this page does not print one. Do not use a figure from a website that does not name its method and source. For how fat percentage is read, see the guide to body fat percentage.

Why hydration and timing change the number

A laboratory study gave 39 adults 2 liters of water and measured them with DXA and impedance before and after. Fat-free mass measured by DXA rose by 1.4 kg in men and 1.7 kg in women. Single-frequency impedance showed 0.5 kg more fat-free mass in men, and both impedance modes read body-fat percentage higher after drinking. The authors say hydration status needs to be standardized for body composition measurements.⁷

That study did not measure muscle itself. It shows that water moves the lean-mass figure on both DXA and impedance, and a lean-mass change is read by many scales as muscle. EWGSOP2 makes the same point: hydration can affect DXA and BIA.¹

1. Pick one time. Weigh at the same time of day, such as first thing in the morning.
2. Hold your routine. Match what you ate and drank before, as far as you can.
3. Use one device. Switching devices changes the reference.²
4. Compare weekly averages. Look at several readings, not one.
5. Add a strength check. Track something a scale cannot, such as reps at a given load.

Steps one to four follow from the hydration and device findings.²⁷ Step five is a practical addition rather than a recommendation from the sources. For the full weighing routine, see the best time to weigh yourself and how accurate smart scales are.

Frequently asked questions