Muscle up: Body composition and health
BMI isn’t the full story - and you know it
If you're training hard and building muscle, chances are your Body Mass Index (BMI) could place you in the “overweight” or even “obese” category — despite having low body fat, strong metabolic health, and excellent fitness. That’s because BMI doesn’t distinguish between muscle and fat.
Instead, a more accurate measure for you may be the Relative Fat Mass Index (RFMI) — a newer, evidence-based metric that uses your waist and height to estimate body fat percentage. RFMI adjusts for gender and gives a more realistic picture for people with high lean muscle mass.
Bottom line: If your RFMI is in the healthy range, you’re likely in great shape — and Thrive sees that.
Why we still talk about BMI
Thrive includes BMI as one of >130 risk indicators because it is still widely used in clinical risk calculations and longitudinal health research. It can be useful to identify risk trends across populations — but it’s not a perfect tool for everyone, especially elite or regular strength trainers.
That’s why we also show your RFMI and this factors strongly into the personalised recommendations that we provide.
We encourage you to track your muscle mass, metabolic markers and connect your wearables to track activity levels, VO2 max and heart rate variability.
Long-term health - Why monitoring still matters
Even if you're strong and metabolically healthy today, risk can change if your routine shifts — such as after an injury, lifestyle change, or as you get older.
Muscle loss accelerates with age, especially after 40–50 years (sarcopenia).
Strength-trained individuals can still develop high blood pressure, blood sugar, or cholesterol without regular monitoring.
Visceral fat can accumulate without obvious physical changes, particularly during periods of reduced training or poor sleep.
Thrive and your training team can help you stay on top of these invisible shifts, so your body doesn’t betray your hard work later.
Thrive supports you — Not just the average
Our mission is to boost the healthspans of as many people as possible.
We’re not here to call fit, strong people ‘unhealthy’. We’re here to support you to be informed about your health —by monitoring other markers and alerting you early if things start to shift.