Arm Muscle Area and Arm Fat Area (MUAC + triceps skinfold)
Regional protein and fat reserves from mid-upper arm circumference and triceps skinfold, without needing a weight
Why use it
It needs no scale and no height. In oedema, ascites, large tumour burden, amputation, or simply in a patient who cannot stand, body weight stops describing tissue mass, and the arm becomes the practical compartment to follow. Serial measurements in one patient detect protein depletion earlier than weight does.
About
From the mid-upper arm circumference and the triceps skinfold, simple geometry separates the arm into a muscle compartment and a fat compartment. Arm muscle area tracks somatic protein reserve; arm fat area tracks the subcutaneous energy reserve. Both are reported here with the total arm area and the arm fat index. In adults the muscle area is corrected for the cross-section of bone and neurovascular tissue, as Heymsfield proposed; in children it is left uncorrected, because that is the form Frisancho's paediatric percentile tables were built on.
When to use
- Nutritional assessment where weight is unobtainable or unreliable
- Serial monitoring of somatic protein during illness, oncology treatment or critical care
- Screening for protein-energy malnutrition in children from one year of age
- Geriatric assessment where height has been lost and BMI is distorted
- Field and resource-limited settings, where MUAC alone is already a triage tool
Formula
Pearls & pitfalls
- Measure at the midpoint between the acromion and the olecranon, on the non-dominant arm, with the arm hanging relaxed; the triceps skinfold must be taken at the same level.
- MUAC alone is a validated triage measure for acute malnutrition in children aged 6-59 months; splitting it into muscle and fat adds the information about which compartment is depleted.
- In oedema the skinfold is falsely thick and the circumference falsely large, so both compartments are overestimated — note the oedema with the result rather than discarding the measurement.
- Arm fat area is more responsive to short-term energy balance; arm muscle area moves more slowly and reflects the protein story.
- The arithmetic assumes the arm is a circle and the fat an even annulus. It is not, which is why the value is for tracking a trend, not for a one-off diagnosis.
Next steps
Compare the result against age- and sex-specific percentiles (Frisancho's tables for children, NHANES-derived tables for adults) rather than reading the absolute area. A falling arm muscle area with a preserved arm fat area suggests selective protein depletion and should prompt a protein-focused intervention and a search for ongoing catabolism.
Evidence
Frisancho (Am J Clin Nutr 1981) published the arm muscle and arm fat area percentile tables derived from the US Ten-State Nutrition Survey and NHANES I, which remain the reference for paediatric interpretation. Heymsfield et al. (Am J Clin Nutr 1982) showed the uncorrected geometric estimate overestimates muscle because it includes bone and neurovascular tissue, and proposed the constant corrections of 10 cm² in men and 6.5 cm² in women.
References
- Frisancho AR. New norms of upper limb fat and muscle areas for assessment of nutritional status. Am J Clin Nutr. 1981;34(11):2540-2545. — link
- Heymsfield SB, McManus C, Smith J, Stevens V, Nixon DW. Anthropometric measurement of muscle mass: revised equations for calculating bone-free arm muscle area. Am J Clin Nutr. 1982;36(4):680-690. — link
- Frisancho AR. Anthropometric Standards for the Assessment of Growth and Nutritional Status. Ann Arbor: University of Michigan Press; 1990. — link
- WHO. Physical status: the use and interpretation of anthropometry. WHO Technical Report Series 854. Geneva: WHO; 1995. — link