The muscle mass index MMI
Authors: Eva Heinen, graduate sports scientist (Univ), Prof. Dr. med. Edgar Heinen
As described in the FFMI chapter, fat-free mass does not only consist of muscle mass – as described in many publications – but also of:
- The fat-free mass of the adipose tissue [see FFMI].
- The mass of the internal organs (without fat)
- The mass of skin (without subcutaneous fatty tissue) and bone (without fatty marrow) [see BMCI].
- As well as the muscle mass (also this without fat mass)
After eliminating the problem of fat-free mass in adipose tissue, we now approach the second parameter of particular interest alongside fat mass: muscle mass: After all, muscle mass is highly variable and of particular interest during training and weight changes, especially when losing weight.
In contrast to the muscles, it can be assumed that the mass of the internal organs, skin and bones (after eliminating the problem of fat storage) does not change significantly in comparison to body weight, neither during training nor when losing weight. It therefore makes sense to combine skin, bonesand internal organsinto one overall parameter (HKIO). Figuratively speaking, you can imagine a person who consists only of skin, bones and internal organs and who no longer has any fat or muscle.
However, it can be assumed that the mass of skin and bones (see bone mass index) and the internal organs also show a dependence on body size, so that the use of an index in the usual way is also appropriate here.
While average weights can easily be found for the internal organs, it is more problematic for the skin, bones and intestines. On the basis of different data, in particular the preparation data of Clarys et al, 1999, the assumption of a mass of 22 kg for skin, bones plus internal organs, each without fat in a 1.70 meter tall man seems appropriate. An appreciable difference in the fat-free masses mentioned for women cannot be assumed. It therefore seems reasonable to assume that a reasonable value for a skin, bone and internal organ index (HKIO index) of 7.6 kg/m² is realistic (22 kg/1.7 m/1.7 m). With this assumption, a quite realistic muscle mass index (MMI) can be calculated from the following formula:
MMI = BMI – FMI – Fat-free mass index of adipose tissue – HKIO index.
The fat-free mass index of the adipose tissue is calculated from the FMI*0.282
The following mean values ± standard deviations for the MMI were determined for the group studied:
Women: 5.0 ± 1.9 kg/m²
Men: 10.6 ± 2.2 kg/m²
Based on these parameters, a woman with an MMI of < 3.1 kg/m² would be described as having weak muscles, while a woman with an MMI of > 6.9 would be described as having strong muscles. Between the two values are the 66.8% who have normal musculature.
In men, an MMI of < 8.4 would be the limit to a weakly developed musculature, an MMI of > 12.8 kg/m² would be considered a strongly developed musculature.
If you have the muscle mass index, you can calculate the muscle mass for each person examined:
Muscle mass = MMI * height² (m²) and check whether there is a correlation between the muscle mass or the MMI and height.
A highly significant (p<0.001) correlation between muscle mass and height was found for both women and men. The difference in muscle mass between men and women is – depending on body size – 12 – 20 kg!
If, on the other hand, the relationship between the muscle mass index and height is examined, no significant correlation can be found for either women or men.
Experience shows that an index is of little use to the user. Instead, they want to know what muscle mass is considered normal, weak or strong for their body size. That is why we have provided you with a table for women and men to download, which allows you to assess the muscle mass (in kg) for the respective body size of the woman or man.
Normal values are usually defined as the mean +/- 2 standard deviations. Given the great variability of muscle mass, only the mean value +/- 1 SD was defined as the normal range; values < – 1 SD are referred to as “muscle-weak”, those > + 1 SD as “muscle-strong”.
The differences between men and women are considerable! An example of the body composition including the musculature for women and men can be found on the summary page: Body composition.
In the chapter on exercise and osteoporosis, we presented an interesting correlation between muscle mass and total body bone mineral content in people under the age of 40.
One problem that has not really been solved to date is edema, water retention, which leads to an incorrectly high muscle mass in practically all routine procedures. Conversely, in the stage of exsiccosis, dehydration, too low a muscle mass is determined (e.g. when taking or abusing diuretic medication, e.g. to achieve weight loss, better to feign).