Imagine a metric that goes beyond weight. Instead, it compares your muscle mass relative to your height to give a fuller picture of your body composition and fitness. It’s not just a short-term fitness fad but a measurement that could change how you monitor your body composition and your progress when you work out. This measurement is called the Relative Muscle Index (RMI).
What is Relative Muscle Index?
When you step on a standard scale, the reading you get back doesn’t differentiate between body fat and muscle, while Relative Muscle Index, or RMI does. It can give you an idea of your composite physical make-up or body composition. It sounds simple, but it has profound implications for how we assess and monitor body composition and health.
Why RMI Is Better
You’ve probably heard all the arguments against using Body Mass Index (BMI) for assessing body composition and health. This measurement that many healthcare practitioners use divides your weight by your height squared:
The basic formula for BMI is:
BMI = weight (kg) / height² (m²)
The downside? This method can’t distinguish between body fat and muscle mass, making it a limiting measure. Plus, it says nothing about the distribution of your body fat. Even healthcare practitioners are frustrated with the concept of BMI. If you only use BMI, you’ll label athletes with lots of muscle as obese and elderly people with muscle loss as healthy.
RMI is different. It homes in on muscle mass. That’s because muscle is important for metabolic health, insulin sensitivity, and your health. When you have more muscle, you have better glucose control, bone density, and a modestly faster metabolism. By measuring muscle mass relative to height, RMI is a better marker of health status.
Measuring RMI: The Science Behind the Numbers
To calculate your RMI, you need to know how much muscle mass you have. The most practical way to measure muscle mass is to use bioelectrical impedance analysis (BIA. This method measures your body composition by passing a weak electrical current through your body. You can also use dual-energy X-ray absorptiometry (DXA) scans. These scans use low-dose X-rays to measure bone density, fat mass, and lean body mass.
What’s the best approach? Research shows BIA has a large individual prediction error. Plus, the readings may vary based on how hydrated you are. To get either, you’ll have to go to a healthcare center or fitness center that offers it. Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) are the gold standard for measuring RMI, but they’re expensive and you get more radiation exposure with CT scans than with a DXA scan.
Once you know your muscle mass, you divide it by height squared (in meters). The resulting number is the RMI.
RMI = Muscle Mass (kg) / Height² (m²)
What distinguishes BMI from RMI is you’re using muscle mass rather than body weight. You don’t have to do these calculations, as there are calculators to help you out. How do you interpret the value? Higher values indicate greater muscle mass relative to height. That’s beneficial for your health. While we need more research for validation, some preliminary studies suggest:
- For men: An RMI below 7.26 kg/m² may indicate sarcopenia risk.
- For women: An RMI below 5.45 kg/m² may indicate sarcopenia risk.
Applications in Fitness and Health Assessment
So, how can knowing your RMI help you in your fitness journey? As experts point out, you can use it in the following ways:
- Tracking Muscle Growth: Use it to track your strength or body building process. It’s a more precise way to determine whether you’re gaining muscle.
- Assessing Sarcopenia Risk: By knowing your muscle mass relative to your height (RMI), you’ll know whether you have sarcopenia, age-related loss of muscle mass. It’ll give you a heads-up on whether you’re heading into the sarcopenic dangers zone. Knowing this, you can adjust your strength training and diet accordingly.
- Fine-Tune Your Nutrition: You can also use RMI to fine tune and optimize your diet for a healthy body composition. Too much protein? Not enough carbs? RMI helps you strike a better balance for your health and body composition.
- Monitoring Your Health: RMI can be a health detective too. Muscle mass is a key contributor to insulin sensitivity and metabolic health. Muscles are metabolic powerhouses, influencing factors like how you process sugar to how quickly you burn calories. Low RMI? It might be waving a red flag about your metabolic health, so you can have a more informed conversation with your doctor.
The Evolving Landscape of RMI Research
Get ready for more exciting developments in RMI. Scientists now need to establish RMI norms across different age groups, ethnicities, and athletic populations. Once we have these norms, your RMI won’t just be a number; it’ll show where you fit on the RMI scale. Scientists also need longer-term studies to show how RMI changes over time and how much these changes affect longevity and health outcomes. Will a high RMI in your thirties be the secret sauce for sprightly seventies? Could changes in your RMI predict health hiccups down the road? That’s what scientists want to know. Maybe one day RMI will replace the current standard of BMI, the measurement most healthcare providers currently use.
Embracing the RMI Revolution
If validated by future studies, RMI could change how we approach fitness and health assessments, making it the fitness indicator you never knew you needed. Welcome to the future of fitness assessment! But also know that RMI is just one health metric. Be sure you’re following other markers of health – lipid levels, blood pressure, inflammatory markers, and lab values that measure metabolic health, liver, and kidney function.
References:
- Buckinx F, Landi F, Cesari M, Fielding RA, Visser M, Engelke K, Maggi S, Dennison E, Al-Daghri NM, Allepaerts S, Bauer J, Bautmans I, Brandi ML, Bruyère O, Cederholm T, Cerreta F, Cherubini A, Cooper C, Cruz-Jentoft A, McCloskey E, Dawson-Hughes B, Kaufman JM, Laslop A, Petermans J, Reginster JY, Rizzoli R, Robinson S, Rolland Y, Rueda R, Vellas B, Kanis JA. Pitfalls in the measurement of muscle mass: a need for a reference standard. J Cachexia Sarcopenia Muscle. 2018 Apr;9(2):269-278. doi: 10.1002/jcsm.12268. Epub 2018 Jan 19. PMID: 29349935; PMCID: PMC5879987.
- Derstine, B.A., Holcombe, S.A., Wang, N.C. et al. Relative muscle indices and healthy reference values for sarcopenia assessment using T10 through L5 computed tomography skeletal muscle area. Sci Rep 14, 21799 (2024). https://doi.org/10.1038/s41598-024-71613-x.
- Borga M, West J, Bell JD, et al. Advanced Body Composition Assessment: From Body Mass Index to Body Composition Profiling. Journal of Investigative Medicine. 2018;66(5):1-9. doi:https://doi.org/10.1136/jim-2018-000722
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