Skeletal Muscle Mass Index (SMMI)
Skeletal muscle mass index (SMMI), also called skeletal muscle index (SMI), measures appendicular skeletal muscle mass (arms and legs) divided by height squared (kg/m²). It is a key diagnostic criterion for sarcopenia (age-related muscle loss), alongside grip strength and physical performance. Data are from Lee et al. 2020, a cross-sectional study of 390,565 White adults aged 40 to 69 from the UK Biobank. Muscle mass was estimated by bioelectrical impedance analysis (BIA). These norms apply to White British/European populations; other ethnic groups may differ (South Asian and Black adults tend to have lower and higher muscle mass respectively at the same BMI). Cutoffs for low muscle mass vary by consensus group. The European Working Group on Sarcopenia in Older People (EWGSOP2) uses <7.0 kg/m² for men and <5.5 kg/m² for women as low-SMMI thresholds.
Data source: Lee et al. (UK Biobank SMMI) About this study
Skeletal Muscle Mass Index (SMMI) Norms by Age and Sex (kg/m2)
| Age | Sex | Percentile | ||||
|---|---|---|---|---|---|---|
| 5th | 25th | 50th | 75th | 95th | ||
| 40-44 | Male | 7.38 | 8.4 | 8.95 | 9.58 | 11.39 |
| Female | 5.75 | 6.41 | 6.8 | 7.3 | 8.96 | |
| 45-49 | Male | 7.29 | 8.32 | 8.88 | 9.52 | 11.37 |
| Female | 5.74 | 6.41 | 6.82 | 7.32 | 9.01 | |
| 50-54 | Male | 7.19 | 8.23 | 8.8 | 9.45 | 11.34 |
| Female | 5.71 | 6.39 | 6.81 | 7.32 | 9.02 | |
| 55-59 | Male | 7.07 | 8.1 | 8.67 | 9.32 | 11.21 |
| Female | 5.68 | 6.37 | 6.78 | 7.29 | 8.94 | |
| 60-64 | Male | 6.95 | 7.97 | 8.53 | 9.17 | 10.99 |
| Female | 5.68 | 6.37 | 6.78 | 7.28 | 8.87 | |
| 65-69 | Male | 6.81 | 7.82 | 8.37 | 9 | 10.72 |
| Female | 5.68 | 6.36 | 6.77 | 7.27 | 8.81 | |
What to expect by age group
Among adults aged 40 to 44, the middle 50% measure 8.4 to 9.6 kg/m² for men and 6.4 to 7.3 kg/m² for women. SMMI declines gradually with each 5-year bracket, with men losing muscle mass at a somewhat steeper rate than women; men carry around 2 kg/m² more appendicular muscle relative to height than women. Scores below 8.4 kg/m² (men) or 6.4 kg/m² (women) are typically below average for this age group; scores above 9.6 kg/m² (men) or 7.3 kg/m² (women) are above average.
| Age | Males | Females |
|---|---|---|
| 40-44 | 8.4 to 9.58 | 6.41 to 7.3 |
| 45-49 | 8.32 to 9.52 | 6.41 to 7.32 |
| 50-54 | 8.23 to 9.45 | 6.39 to 7.32 |
| 55-59 | 8.1 to 9.32 | 6.37 to 7.29 |
| 60-64 | 7.97 to 9.17 | 6.37 to 7.28 |
| 65-69 | 7.82 to 9 | 6.36 to 7.27 |
Detailed Breakdowns
Select an age group and sex below for detailed percentile charts, tables, and ratings.
Frequently Asked Questions
What is skeletal muscle mass index (SMMI)?
SMMI is appendicular skeletal muscle mass (the combined muscle in your arms and legs) divided by height squared (kg/m²). It normalises muscle mass for body size, making it comparable across individuals of different heights.
How is SMMI measured?
The most common methods are dual-energy X-ray absorptiometry (DXA) and bioelectrical impedance analysis (BIA). DXA is considered the gold standard; BIA (used in Lee et al. 2020) is less precise but practical for large-scale studies. Values may differ slightly between methods.
What SMMI indicates sarcopenia?
The European Working Group on Sarcopenia in Older People (EWGSOP2) defines low muscle mass as SMMI below 7.0 kg/m² for men and 5.5 kg/m² for women. The Asian Working Group for Sarcopenia uses lower cutoffs (7.0 and 5.7 kg/m²) but note that these thresholds were developed for DXA, and BIA-based values require separate validation.
Why are these norms limited to ages 40-69?
The Lee et al. 2020 study covers the UK Biobank cohort, which at the time of analysis was predominantly aged 40-69. No data outside this range are available from this source.
Do these norms apply to all ethnic groups?
No. The UK Biobank is predominantly White British. Research shows that South Asian adults tend to have lower appendicular muscle mass at the same BMI, while Black adults tend to have higher values. Separate reference ranges would be more appropriate for non-White populations.
Why are P5 and P95 approximate?
Lee et al. 2020 reports centile curves at C2, C25, C50, C75, and C98. We use C25, C50, and C75 directly as P25, P50, and P75. C2 is used as a proxy for P5, and C98 as a proxy for P95. C2 and C98 are not exactly equal to P5 and P95, they represent the 2nd and 98th centiles, which are slightly more extreme. The difference is small in absolute terms but should be noted.