Your skeleton muscle age is calculated based on BMI-adjusted SMA, or SMA/BMI, where SMA covers all the muscles captured in an axial CT image at the L3 lumbar vertebrae level and demarcated in the range of −29 to +150 Hounsfield units, and BMI stands for Body Mass Index, defined as BMI = weight (kg) / height (m²).

To estimate a skeleton muscle age with the data in Kim et al. (2021), we first regressed SMA/BMI on a constant and Age with a sample of the subjects aged between 35 and 65, where the lower and upper bounds of Age are approximately the 5th and 95th percentiles, respectively, for both sex. Then, we found the inverse of the regression line by solving it for Age, which expresses Age as a function of SMA/BMI. We computed the standard error of the estimated Age using the delta method on the original variance-covariance matrix adjusted for heteroscedasticity. Finally, we obtained the skeleton muscle age function separately for men and women by running the regression separately.

Your Skeleton Muscle Age Is:


Average BMI-Adjusted SMA over Age Groups


This page provides visual summary of Skeleton Muscle Area (SMA) by age groups in the South Korean population, plotting sample averages along with the reference lines of twice sample standard deviation. It also presents three additional values of SMA, adjusted on height, weight, and Body Mass Index (BMI). The sample data are those employed in Kim et al. (2021) that investigates abdominal CT scans of 11,845 people (7,314 men and 4,531 women) taken for routine health check-up at Asan Medical Center in Seoul, South Korea over the period of January to December 2012.









Kim, Eun Hee, Kyung Won Kim, Yongbin Shin, Jiwoo Lee, Yousun Ko, Ye-Jee Kim, Min Jung Lee et al. "Reference data and T-scores of lumbar skeletal muscle area and its skeletal muscle indices measured by CT scan in a healthy Korean population." The Journals of Gerontology: Series A 76, no. 2 (2021): 265-271. [Link to the full paper]


Although computed tomography (CT) is considered the gold standard for investigating skeletal muscles, diagnostic cutoff points for sarcopenia have not been established. We therefore suggested clinically relevant diagnostic cutoff points for sarcopenia based on reference values of skeletal muscle area (SMA) measured by CT scan in a large-sized healthy Asian population.


This cross-sectional analysis included 11,845 subjects (7,314 men, 4,531 women) who underwent abdominal CT scans in South Korea. SMA including all muscles on the selected axial images of the L3 lumbar vertebrae level was demarcated using predetermined thresholds (−29 to +150 Hounsfield units). SMA indices (height-, weight-, and body mass index [BMI]-adjusted) were calculated.


When T-score < −2.0 was used as the cutoff for defining sarcopenia, the sex-specific cutoff points of SMA, SMA/height² , SMA/weight, and SMA/BMI were 119.3 and 74.2 cm², 39.8 and 28.4 cm²/m², 1.65 and 1.38 cm²/kg, and 4.97 and 3.46 in men and women, respectively. In both sexes, the SMA/BMI values peaked in the 20s and decreased gradually. The SMA/BMI yielded the highest diagnostic rate of sarcopenia (4.2% in men, 8.7% in women), while SMA/height² provided the lowest yield (2.8% in men, 1.0% in women).


This is the first study to report the reference values of SMA and skeletal muscle indices (SMIs) measured on CT scans and to suggest cutoff points for diagnosis of sarcopenia based on T-score in Asian subjects. BMI-adjusted index (SMA/BMI) was the best index of CT-measured SMA to reflect the age-related muscle changes and to maximize the diagnostic yield for sarcopenia.