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Use body composition as context for performance. FFMI is sensitive to measurement assumptions and should not be turned into a universal natural limit or an accusation about a named athlete.
Know what the index counts
Fat-free mass is body mass minus estimated fat mass. FFMI divides that fat-free mass by height squared. Fat-free mass includes water, bone and organs as well as skeletal muscle, so the result is not a direct measurement of muscle quantity. The original height-normalised version adds 6.3 × (1.80 − height in metres). Distinguish raw and normalised FFMI when comparing values; otherwise two apparently identical numbers may have been calculated differently. [1]
Why the number 25 needs context
Kouri and colleagues studied 157 men, including 83 reported anabolic-steroid users and 74 nonusers. The nonuser group’s normalised values reached approximately 25, while higher values occurred among users. The paper also discussed historical bodybuilders using estimates. This influential observation was not a census of all genetically possible physiques, a validated individual doping test or a model for women and every sport. It cannot establish that a specific armwrestler is using drugs from a photo, height and reported body weight. [1]
The same athlete, three body-fat assumptions
| Assumed body fat | Fat-free mass | FFMI |
|---|---|---|
| 12% | 79.20 | 24.44 |
| 15% | 76.50 | 23.61 |
| 18% | 73.80 | 22.78 |
FFM = 90 * (1 - body_fat/100); FFMI = FFM / 1.8**2
A small body-fat error changes the conclusion
Consider an illustrative athlete weighing 90 kg at a height of 1.80 m. At 15% estimated body fat, fat-free mass is 76.5 kg and FFMI is 23.61. Changing only the body-fat estimate to 12% produces 24.44; changing it to 18% produces 22.78. These are calculations, not three measured athletes. The example shows how uncertainty in an input can move the result substantially before any real muscle gain or loss has occurred.
Return the discussion to the table
Armwrestling performance depends on specific force production, timing, geometry and technique, not simply total fat-free mass. Comparisons between armwrestlers and other strength-trained athletes support looking at specific performance characteristics. Neither FFMI nor a chosen body-fat percentage reveals how much muscle someone can add to the forearm, how much leg mass should be removed, or an ideal competitive body weight. Track body mass alongside repeatable sport-relevant tests; a weight increase is useful only in relation to the athlete’s goals and constraints. [2, 3]
What this does not tell us
The calculations depend on estimated body fat and accurate height. This is an explanatory example, not a body-composition assessment, weight-cutting plan or judgement about any person’s drug use. Fifteen percent body fat is an input here, not a universal optimum.
References
Based on the supplied English video transcript and the sources below.
- Kouri EM, Pope HG Jr, Katz DL, Oliva P. (1995). Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine. 5(4):223–228.
- Coletta F, Cesanelli L, Kamandulis S, Conte D. (2025). Comparative Analysis of Elbow Flexor Morphology, Physiology, and Performance Between Arm Wrestlers and Strength-Trained Athletes. Journal of Strength and Conditioning Research. 39(5):579–586.
- Morton RW, et al. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine. 52:376–384.
