Anthropometric measurement of muscle mass: revised equations for calculating bone-free arm muscle area

Abstract

Arm muscle area (AMA, cm²) is currently calculated from triceps skinfold thickness (TSF, cm), and midarm circumference (MAC, cm). In assessing the accuracy of the current equation by comparison to AMA measured by computerized axial tomography, error in each of the four approximations made was found to result in a 20 to 25% overestimate of AMA. Two correctible error sources were: a 10 to 15% overestimation caused by assuming a circular midarm muscle compartment and a 5 to 10% overestimation due to inclusion of midarm cross-sectional bone area. Corrected AMA equations for men and women were respectively: [(MAC − π × TSF)²/4π] − 10, and [MAC − π × TSF)²/4π] − 6.5. With two additional study groups, the overall improved accuracy of the new equations was confirmed, although the average error for a given patient was 7 to 8%; the relationship between corrected AMA and total body muscle mass was established [muscle mass (kg) = (ht, cm²) (0.0264 + 0.0029 × corrected AMA)]; and the minimal range of corrected AMA values compatible with survival (9 to 11 cm²) was defined. Bedside estimates of undernutrition severity and prognosis can therefore be calculated from two simple measurements, TSF and MAC.