Skeletal Muscle Mass from Limb Anthropometry (Lee / Poortmans)
Total-body skeletal muscle mass and muscle index from arm, thigh and calf girths corrected for skinfolds — adult and paediatric equations
Why use it
Skeletal muscle is the tissue that carries function, and it is not what fat-free mass measures. In sarcopenia, in cancer cachexia and in recovery from critical illness, muscle can fall while fat-free mass is held up by water. Where MRI, DXA or bioimpedance are unavailable, limb anthropometry is the field method with the best validation against imaging.
About
Estimates total-body skeletal muscle mass from the circumferences of the arm, thigh and calf, each corrected for the overlying skinfold, together with height, age and sex. Lee's equation was derived in adults against whole-body MRI; Poortmans' paediatric equation was derived from it and validated against DXA in children and adolescents aged 7 to 16. The calculator also reports the skeletal muscle index, which normalises the mass for height and is the form used in sarcopenia criteria.
When to use
- Sarcopenia assessment where DXA or bioimpedance is unavailable
- Following muscle during rehabilitation, oncology treatment or recovery from critical illness
- Sports and exercise assessment, where change in muscle rather than weight is the goal
- Paediatric and adolescent body-composition research and specialist clinics
Formula
Pearls & pitfalls
- The skinfold correction is what makes this work: an uncorrected girth measures muscle plus the fat ring around it, and in an adipose limb the fat can be most of the difference.
- Take the girths at standard landmarks with the muscle relaxed, and the skinfold at the same level on the same limb, or the correction subtracts the wrong ring.
- The adult equation carries an ethnicity term because Lee's derivation found systematic differences; it is small but it is part of the published equation, not an optional extra.
- Skeletal muscle index, not the absolute mass, is what sarcopenia criteria use — a small person with low muscle mass may have an entirely normal index.
- The adult and paediatric equations are not continuous at the boundary. Expect a step if you run a 16- and a 17-year-old through the two forms; use the trend within one equation, not across both.
Next steps
A low skeletal muscle index should be read alongside a measure of function — grip strength and gait speed — because the current sarcopenia definitions require low function as well as low mass. In children, interpret against age- and sex-specific reference values rather than adult thresholds.
Evidence
Lee et al. (Am J Clin Nutr 2000) derived and cross-validated the equation against whole-body MRI in 244 adults, reporting R² of about 0.91 with a standard error near 2.2 kg. Poortmans et al. (Med Sci Sports Exerc 2005) adapted it for 7-16-year-olds, validating against DXA and 24-hour urinary creatinine in 39 children and adolescents with R² of about 0.97 for the anthropometric method.
References
- Lee RC, Wang Z, Heo M, Ross R, Janssen I, Heymsfield SB. Total-body skeletal muscle mass: development and cross-validation of anthropometric prediction models. Am J Clin Nutr. 2000;72(3):796-803. — link
- Poortmans JR, Boisseau N, Moraine JJ, Moreno-Reyes R, Goldman S. Estimation of total-body skeletal muscle mass in children and adolescents. Med Sci Sports Exerc. 2005;37(2):316-322. — link
- Cruz-Jentoft AJ, Bahat G, Bauer J, et al. Sarcopenia: revised European consensus on definition and diagnosis (EWGSOP2). Age Ageing. 2019;48(1):16-31. — link