AC
Arm Circumference and Arm Muscle Circumference (AC, AMC)
Arm Circumference and Arm Muscle Circumference are estimates of upper-limb dimensions that the manual uses as simple nutritional indicators.
Short answer
Arm circumference and arm muscle circumference are orientational estimates of limb size. The manual gives no reference range for them, so their value lies mainly in following your own development — and even there segmental lean mass is more precise.
What the value represents
Arm Circumference (AC) describes the whole circumference of the upper limb. It consists of muscle, subcutaneous fat and skin.
Arm Muscle Circumference (AMC) separates out the part accounted for by muscle. The manual describes it as the circumference of the muscle in the mid-upper arm.
On InBody both values are estimates from body composition, not tape measurements.
The manual classes them as nutritional indicators: arm circumference is described as proportional to muscle mass and used to assess protein status. It recommends following changes through repeated measurement.
- The circumference of the arm in centimetres and its muscle component.
- A simple anthropometric indicator for orientational tracking of nutritional status and upper-limb muscle.
- It does not represent muscle strength or performance.
How the value is produced
- An estimate. Both the manual and the repository reference class arm circumference and arm muscle circumference among estimated values derived from body composition.
- The gap between the two values corresponds to the layer of fat and skin. Arm muscle circumference is therefore less sensitive to changes in fat than the total circumference.
- The manual gives neither the calculation method nor a reference range for these circumferences.
How to read the current value
- The value is meaningful chiefly as a starting point for tracking. The manual gives no reference range, so there is nothing to compare it against.
- Compare the two values with each other. A larger gap means a higher share of fat and skin, a smaller gap a more muscular arm.
- The number will not match what you measure with a tape. It is an estimate derived from body composition.
- For assessing arm muscle, segmental lean mass, also on the result sheet, is more specific.
How to read change over time
- A rise in arm circumference with strength training makes sense over months, not weeks.
- Distinguish what is growing: when the total circumference grows and the muscle circumference holds, fat is being added. When both grow, the muscle component is growing too.
- Short-term perfusion of the muscle after training affects the actual arm circumference; the InBody estimate moves with the body composition estimate.
- If you are following one arm after an injury or during rehabilitation, keep to the same side and the same measurement conditions.
General principles: how to read change between measurements →
What to read it with
- Segmental lean mass of the arms – more precise information about upper-limb muscle.
- Segmental fat – the fat component of the same segment.
- Skeletal Muscle Mass – the overall context.
- Body circumferences – right and left arm circumference from the circumference section.
- Hand grip strength – a functional measure a circumference cannot replace.
Common misunderstandings
- This is not a tape measurement but an estimate from body composition.
- A larger arm circumference does not automatically mean more muscle; it can be fat or fluid retention.
- Arm muscle circumference is not the same as arm muscle mass in kilograms.
- A circumference says nothing about strength. That is measured differently, with a dynamometer for instance.
What can affect the result short term
- Hydration, food and drink before the measurement.
- Training shortly beforehand and temporary muscle perfusion.
- Time of day and fluid shifts within the body.
- Swelling or inflammation in the limb.
What to do next
- If you are following your arms, use segmental lean mass above all; treat circumferences as a companion.
- When you want to compare actual centimetres, measure your arm with a tape yourself — always at the same place and on the same side.
- Address a difference between the right and left side in the context of segmental analysis rather than from the circumference alone.
What the value does not determine
- This is not a tape measurement but an estimate from body composition.
- The manual gives neither a reference range nor the calculation method.
- It says nothing about muscle strength or limb function.
- The manufacturer internal validation study against DEXA did not cover circumferences.
On terminology
- AC (Arm Circumference) is the total arm circumference, AMC (Arm Muscle Circumference) its muscle part. On the result sheet these are two separate values of the same concept.
- Right and left are given from the point of view of the person being measured, not the observer.
In more depth, with sourcesIn more depth: why arm circumference is used in nutritional assessmentExpand the detailed explanation
The mid-upper arm is popular in anthropometry because the muscle and fat compartments sit alongside each other there and the circumference is easy to repeat. The manual states that arm circumference is proportional to muscle mass and used to assess protein status, while arm muscle circumference describes the muscle part itself in the middle third of the arm. How InBody calculates the two values is not described; in classification terms they are estimates derived from the body composition analysis.
Because both circumferences come from the same model as segmental lean mass, they are not independent of it and will move together. The difference lies in how well documented they are: segmental lean mass has a correlation of r ≥ 0.95 for all five segments in the manufacturer internal validation study against DEXA, while the circumferences were not verified in that study. For following limb muscle it is therefore more direct to work with segmental lean mass and use circumferences as a companion.
The ESPEN clinical guidelines recall that bioimpedance estimates are sensitive to altered fluid distribution. For limbs that holds especially: swelling, inflammation or temporary perfusion after exertion shift both the actual circumference and the estimate. For comparability the usual rule applies — same conditions, same side, and a sufficient interval between measurements.
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)
On this page
Related values
Related articles
Where to find it
The Visceral Fat Analysis, circumferences and basal metabolic rate.
- 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.
Why did I get a different arm circumference from the one I measured with a tape?
Because the device does not measure the circumference but estimates it from body composition. The difference is therefore not an error. If you want exact numbers, keep your own tape measurements separately.
Which of the two values is better for following muscle?
Arm muscle circumference is less affected by the amount of fat than the total circumference, so it is the more suitable of the two for this purpose. The most direct information about arm muscle, though, comes from segmental lean mass.
Does a larger arm circumference mean more strength?
Not necessarily. The amount and the function of muscle are related but not the same. Strength shows only in a measurement, such as hand grip strength.
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.