Value comparison
SMM vs. soft lean mass vs. fat free mass
Short answer
The result sheet shows three "muscle" numbers side by side — skeletal muscle, lean mass without bone minerals, and everything except fat — and each of them covers a different slice of the same body.
Side by side
| Aspect | Muscle massSMM | Soft lean massSLM | Fat free massFFM |
|---|---|---|---|
| What it includes | The muscle between bones and joints responsible for voluntary movement. The narrowest of the three. | Mass without fat and without bone minerals — body water, protein and non-bone minerals. | Everything except fat: body water, protein and minerals including bone. |
| How it is produced | Calculated; the InBody manual gives neither an equation nor the exact relationship to soft lean mass. | Calculated: SLM = FFM − bone mineral content. | Calculated: FFM = total body water + protein + minerals. |
| Unit | kg | kg | kg |
| Where on the sheet | Body Composition Analysis and the Muscle-Fat Analysis chart, where it is compared with weight and fat mass. | Body Composition Analysis, plus the tabular Muscle-Fat Analysis on the body water sheet. | Body Composition Analysis, the summed bar just below weight. |
| What it is read for | Muscle development and its change with training. The manual states that 100 % means enough muscle for the ideal weight. | A practical middle step: lean tissue without bone, useful for soft tissue and nutritional status. | The basis for basal metabolic rate and the reference frame of other values; it includes bone. |
| When to use it | For the question "am I gaining muscle?" over time in the same person. | For comparison with values that exclude bone. | For anything derived from fat free mass, typically basal metabolic rate. |
| What it cannot tell you | It cannot say which segment lacks muscle — that is what the segmental analysis is for. | It cannot separate muscle gain from retained water; check the ECW/TBW ratio alongside it. | It says nothing about muscle quality or distribution, and it includes bone minerals. |
When the values diverge
- After a training block all three rise, but skeletal muscle mass is the narrowest of them, so its gain in kilograms is the smallest and fat free mass shows the largest one.
- After fluid retention or swelling soft lean mass and fat free mass jump fastest, because both contain all body water. Skeletal muscle mass is derived from the same measurement, so it can be overstated too.
- When comparing with another device or a study it is easy to compare unlike things: DEXA "lean mass" corresponds to fat free mass rather than to skeletal muscle mass.
The most common wrong conclusion
Adding the values up or treating them as three different muscles. They are three nested slices of the same body: skeletal muscle sits inside soft lean mass, which sits inside fat free mass.
Full explanations of each value
Related articles
An InBody measurement is not a diagnostic tool. Results do not determine health status and do not replace a medical examination.
FAQ
Frequently asked questions
Which of the three is "my muscle mass"?
Muscle in the everyday sense is skeletal muscle mass (SMM). Soft lean mass and fat free mass are broader categories that also contain organs, body water and — in the case of fat free mass — bone.
Why did fat free mass rise more than skeletal muscle after a training block?
Fat free mass carries all of your body water, so better hydrated muscles and restored glycogen stores show up in it. Skeletal muscle mass is a narrower slice and moves more slowly in kilograms, which is why the trend needs several measurements rather than a single difference.
Can I add these values together?
No. They are not separate parts of the body but three nested slices of the same one: skeletal muscle sits inside soft lean mass, which sits inside fat free mass. Adding them would count the same tissue several times.
Explanations make the most sense alongside your own trend.
The public guide is open to everyone. The client portal adds your own value, history and the context of a specific measurement.