Arm anthropometry is used as a proxy measure of body composition (e.g. muscularity, fat free mass and fat mass) by assessing the shape of the upper arm in both clinical and field settings.
Arm anthropometry includes:
These measurements are used to derive the following indices of body composition:
A non-stretchable insertion tape is typically used to assess MUAC and arm length. The tapes are generally graduated in millimetres.
Some tapes also have inbuilt MUAC cut off values to assess nutritional status and are colour coded (red indicates severe acute malnutrition, orange - moderate acute malnutrition, yellow - acute malnutrition and green - well nourished). Not all tapes include the colour orange.
These particular tapes vary according to the population under investigation (e.g. infants, children, adults or older adults).
If deriving the arm anthropometry indices, a skinfold caliper is used to assess the triceps skinfold (TSF).
Figure 1 An insertion tape with three colour codings.
If deriving the arm anthropometry indices, the triceps skinfold (TSF) is also measured with a skinfold caliper at the same midpoint of the arm.
MUAC in isolation is commonly used in nutrition surveillance and screening programs to quickly assess nutritional status. Typically, it is used for the detection and referral of individuals with acute malnutrition.
Estimation of MUAC is important for assessing chronic energy deficiency, but it has also been found to be a useful screening tool for obesity, particularly in children and adolescents.
MUAC is also implemented to predict BMI categories. For instance, it is frequently used in older individuals or in hospitalised patients when weight combined with height, or arm-span or half-span cannot be derived.
MUAC can be used in combination with other arm anthropometry such as triceps skinfold (TSF) to derive the arm anthropometric indices arm muscle area (AMA), arm fat area (AFA), and arm fat index (AFI). These are used for the assessment of regional and total fat mass and fat free mass in resource-limited settings, in hospital settings (in acute disease) or field research settings, where current reference methods of body composition are not feasible.
These indices have been used to evaluate nutritional status and nutritional interventions in patients. Other applications include their use in the interpretation of nutritional status of populations with regard to normal reference curves in research settings.
Measurements of MUAC require very little data processing to derive final estimates of body composition. Estimates should be quality checked during data processing in the same manner as other health related variables, for example by checking for outliers and data entry errors.
MUAC is recorded in mm and this measure equals muscle + bone + fat, as shown in Figure 2.
Figure 2 Illustration of components of a measure of mid-upper arm circumference.
Source: MRC Epidemiology Unit.
MUAC can be used to predict BMI to assess nutritional status (e.g. underweight and overweight) in different ages.
A MUAC of <235 mm -> BMI is likely to be <20 kg/m2
A MUAC of >320 mm -> BMI is likely to be >30 kg/m2
There are cut-off values for children, adults and older individuals, which correspond to BMI categories. Cut-offs are age, sex and population dependant.
Cut-offs in older individuals:
MUAC in the under five year old's, can also be interpreted by comparison against population standards (e.g. WHO standards for arm circumference by age; see www.who.int and WHO Technical Report 2 Source: [ ])
Calculation of arm anthropometric indices
MUAC and TSF are converted from millimetres to centimetres.
The following are the most widely used equations assuming the arm has a cylindrical shape.
Where MUAC and TSF are in centimetres and TUA (total upper arm area), AMA and AFA are in square centimetres and AFI as a percentage.
Equations containing correction factors for bone contributions (humerus) are also available.
The most widely used are reported by Heymsfield et al. (1982). These equations subtract mid-arm bone area (using the constant value of 10 cm2 in men and 6.5 cm2 in women) from the original equations on the basis that bone does not atrophy in proportion to muscle in severe wasting, and that the original equation tends to overestimate AMA when compared with CT values.
The arm index, arm circumference-for-age is used as an alternative indicator of nutritional status when the collection of length/height and weight measurements is difficult, as happens in emergency humanitarian situations due to famine or refugee crises.
This index is expressed in percentiles (percentage of median) and can be assessed by the percentile point achieved by a child relative to the healthy children of that age and gender in the same population. Median is regarded as a reference value, and 3rd and 97th97th percentiles as thresholds to indicate abnormally low or abnormally high values.
Arm circumference-for-age index can also be expressed as Z-score derived by using the formula:
(Measured value – Average value in the reference population) / Standard deviation of the reference population
The WHO growth standards (2006) for arm circumference-for-age are used for interpretation in infants between 3 months and 5 years.
An overview of the arm anthropometry method is outlined in Table 1.
Table 1 Characteristics of arm anthropometry methods.
|Number of participants||Large|
|Researcher burden of data collection||Low|
|Researcher burden of coding and data analysis||Low|
|Risk of reactivity bias||No|
|Risk of recall bias||No|
|Risk of social desirability bias||No|
|Risk of observer bias||Yes|
|Suitability for field use||High|
|Participant literacy required||No|
Considerations relating to the use of the arm anthropometry method in specific populations are described in Table 2.
No guidelines currently exist for the measurement of individuals with physical disabilities, MUAC measurements may be misleading in cases where upper arm muscle might build up to aid mobility.
Table 2 Use of arm anthropometry methods in different populations.
|Infancy and lactation||Suitable, but not recommended for use among infants aged below 3 months due to lack of reliability data; For children aged 6-60 months, adjusting MUAC by calculating the Z-score or adjusting for height does not improve MUAC predictive value. Distraction techniques such as toys, bubbles help occupying the children during the measurement.|
|Toddlers and young children||Distraction techniques such as toys, bubbles help occupying the children during the measurement.|
|Older Adults||The mid-upper arm muscle area calculation may not be valid in obese or older individuals.|
|Other (e.g. Obesity)||The mid-upper arm muscle area calculation may not be valid in obese or older individuals.|
* TOBEC has been used mainly to monitor changes in body composition in women during pregnancy or lactation, in infants and in childhood obesity.
Refer to section: practical considerations for objective anthropometry
A method specific instrument library is being developed for this section. In the meantime, please refer to the overall instrument library page by clicking here to open in a new page.