When it comes to assessing our fitness and health, we are often content to rely on our weight and body mass index (BMI). Yet these indicators provide only a partial view of our body composition.
In fact, they do not distinguish between fat mass and muscle mass. To get a better idea of how much muscle you have, there's another index you need to know: the Fat Free Mass Index (FFMI)(1).
Let's take a closer look:
- what IMM is,
- how it differs from BMI,
- what's in it for him.
We'll then look at how to calculate it yourself, and how to interpret the result.
You'll have all the keys you need to assess your muscular capital and know how to optimize it for your well-being and performance.
| Subject | Details |
|---|---|
| 📊 IMM vs IMC | Specifies non-fat mass, unlike BMI |
| 🧮 IMM calculation | Lean mass (kg) / height² (m) + adjustment |
| 🔬 Why IMM? | Assesses actual body composition |
| 🛠️ Usefulness of IMM | Muscle mass monitoring, training adjustments |
| 👟 Optimization | Strength training, protein nutrition |
| 🚴 Sportsmen | High IMM desirable for performance |
| 👵 Sarcopenia | Detecting muscle loss in the elderly |
| 🔍 Measurement methods | MRI, adipometer, impedancemetry |
| 🧪 Interpretation | Reference tables by age and gender |
| 🥇 Ideal IMM | Varies with age, gender and health/performance goals |
What is the Lean Mass Index?
The Lean Mass Index (LMI) represents the total amount of body mass other than fat of an individual, in relation to his or her height. Note that it includes all muscle mass, but is not solely composed of the latter. It also includes skin, blood, skin, bones, etc.
The BMI differs from the Body Mass Index (BMI), which simply compares weight to height, without taking body composition into account. Thus, a person with a lot of muscle but little fat will have a high BMI, while his or her BMI will be optimal.
Conversely, a person with low muscle mass and high fat mass may have a normal BMI but a poor MMI.
The formula for calculating the lean mass index is as follows:
IMM = lean mass (in kg) / height² (in m) + 6.3 x (1.8 - height)
To calculate your lean body mass, subtract your body fat from your total weight.
For example, a 70 kg person with 20% body fat will have a lean mass of 56 kg (70 - (70×20%)). If she is 1m70 tall, her IMM will therefore be 56 / (1.7×1.7) + 6.3 x (1.8 - 1.7) = 20.
Later on, we'll look at how you can easily obtain your body fat percentage to calculate your BMI. But first, let's take a look at why it's important to know this index.
Why calculate your lean mass index?
Knowing your IMM is essential for several reasons:
- Evaluate your actual body composition, not just your weight. A high body weight may be due to high muscle mass (in the case of athletes) or to excess body fat. The BMI can help you distinguish between the two.
- Track changes in muscle mass over time, whether muscle gain through training or muscle wasting due to aging (sarcopenia). The part of the IMM that includes skin, bone and blood is not subject to change. If the MMI does change, it is mainly due to changes in muscle mass.
- Adapt your training and diet to your level of muscle mass. For example, a low MMI will require a higher protein intake and specific weight training.
- Identify possible sarcopenia, i.e. excessive loss of muscle mass that can occur with age and impact health.
- Preserve muscle mass, a key factor in health, longevity and physical and sporting performance.
To sum up, muscle mass is an essential bodily component that it is vital to know about and maintain throughout life, through regular physical activity and a balanced diet rich in protein.
On this subject, these contents can help you go deeper into the question:
A complementary index to BMI
The BMI complements the Body Mass Index (BMI), which simply relates weight to height without distinguishing between different body tissues.
The IMM therefore enables us to refine the diagnosis by focusing specifically on non-fat mass, in other words lean mass, which is mainly made up of skeletal muscle, but also bone, skin and blood.
How do you determine your MMI?
To calculate your lean mass index, you first need to know your body fat percentage. We can cite 3 main methods:
- MRI (Magnetic Resonance Imaging): mainly used in medical settings, due to its high cost.
- Skinfold calipers (adipometers ): the thickness of subcutaneous adipose tissue is measured at various points on the body (abdomen, arms, legs, etc.). This measurement is then converted into fat mass percentage using prediction equations. This method is subject to variability depending on the clinician, and the distribution of fat (subcutaneous vs. visceral) can be very different between subjects.
- Bioelectrical impedancemetry (impedance scale): this method involves passing a very low-intensity electrical current through the body. The measured resistance (impedance) is used to estimate body water content, which in turn correlates with lean body mass. Depending on the device, variability can be significant and accuracy average. Be sure to follow up with the same device.
These methods are ideal for tracking trends over time, but lack the precision needed for absolute value comparisons.
Once you've obtained your body fat percentage, simply subtract it from 100 to obtain your lean mass percentage. For example, a person with 20% body fat will have 80% lean mass.
We can then calculate the lean mass in kg by multiplying this percentage by the total weight:
Lean mass = weight x (100 - % body fat) / 100
= 70 kg x (100 - 20) / 100 = 56 kg
Finally, the MMI is calculated very simply by dividing this lean mass by height squared, with a correction factor :
IMM = lean mass (kg) / height² (m) + 6.3 x (1.8 - height)
For our example: IMM = 56 kg / (1.7m)² + 6.3 x (1.8 - 1.7m) = 20
Reference tables arethen used to interpret these results according to age and gender.
Interpreting your lean mass index: reference values
Here are the average IMM standards by age and gender:
| Age | Women | Men |
|---|---|---|
| 20-39 years | 15,5 - 17,5 | 18,5 - 20,5 |
| 40-59 years | 15,0 - 17,0 | 18,0 - 20,0 |
| 60-79 years | 14,5 - 16,5 | 17,5 - 19,5 |
An MMI below these values indicates a tendency to have insufficient muscle mass, while a higher MMI may reflect strong muscularity.
IMM ideal for health
Numerous studies have shown that an MMI of between 17 and 20 kg/m² for women and between 20 and 23 kg/m² for men is associated with a lower risk of all-cause mortality.
These values therefore seem optimal for preserving overall health and longevity, by avoiding both excessively low muscle mass (sarcopenia) and excessively high fat mass.
IMM ideal for performance
For performance-oriented athletes, a higher MMI is generally desirable. An MMI greater than 19 kg/m² for women and 22 kg/m² for men is considered ideal for optimizing muscular strength and power.
Of course, these target values need to be adapted according to the type of sport. For example, strength sports such as weightlifting will require a higher MMI than endurance sports like marathon running.
IMM for top athletes
For top-level athletes, the Lean Mass Index (LMI) is a much more relevant indicator than the classic BMI. Indeed, their large muscle mass often pushes their BMI into the overweight or obese zones, which is misleading. Once again, more than an absolute value, this index is particularly interesting for tracking changes in the same athlete over time.
The IMM target values to aim for to optimize performance vary according to the sport practised.
An IMM that is too low may indicate a lack of muscle mass that is detrimental to performance. Conversely, an excessive MMI may reflect an excess of fat mass detrimental to endurance. Conversely, an MMI over 25 has been identified as an indicator of anabolic steroid use.
Regular monitoring of the IMM is essential for athletes in order to :
- Adjust strength training to build muscle mass
- Control body composition during lean periods or weight gain
- Optimize your fitness weight by aiming for the ideal IMM for your discipline
Regularly monitoring your LMI also enables you to measure your progress during weight gain or weight loss. If you gain weight while maintaining or increasing your lean mass index, your weight gain is going well and limiting fat gain.
In contrast, in the dry phase, the main objective is to reduce body fat while preserving muscle mass as much as possible. As a result, you aim to keep your lean mass index (LMI) as high as possible, while losing weight.
This means that while the weight on the scale may decrease as a result of fat loss, your muscle mass as a percentage of your total weight should at least remain stable if you manage to effectively preserve your muscle mass during the dry period.
By combining IMM monitoring and segmental analysis, athletes have interesting tools at their disposal to adjust their physical preparation and achieve the optimal body composition for their high-level performance.
Optimizing your IMM
The key to increasing your MMI is to combine the right weight training with a diet rich in quality protein. Here are a few tips:
- Practice 2 to 4 muscle-strengthening sessions per week targeting the major muscle groups
- Aim for a protein intake of 1.6 to 2.2 g/kg bodyweight per day, depending on your goal. A good dietary program can help you achieve this.
- Focus on complete, high-quality protein sources (sustainable or organic farming, as locally as possible): meat, fish, eggs, dairy products, legumes... If necessary, consider a low-calorie whey (for weight loss) or a high-quality gainer (for weight gain).
- Divide your protein intake over the day into 4 to 6 portions
- Stay well hydrated and respect rest periods for muscle recovery
Regular monitoring of your IMM will enable you to adjust your program according to changes in your muscle mass.
Lean body mass index is a valuable indicator to monitor in addition to weight and BMI.
Easy to calculate yourself from your body fat percentage, it gives a complete picture of your body composition.
Preserving muscle mass is essential at any age, whether to stay healthy, age gracefully or optimize sporting performance. With the IMM, you can adjust your training and nutrition to achieve and maintain optimal muscle levels.
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