FFMI & the Muscular Limit

If you’ve been in the natural bodybuilding world long enough you typically begin to get curious as to what your natural potential is, and by doing so, you also get curious about other people’s state of “natural-ness”. On the flip side, people who are lean and fit have found the Body Mass Index (BMI) extremely inaccurate in determining their health category. In both instances, a type of solution was formulated that could help shed some light and fill in some of the holes in one’s curiosity – that solution is called the Fat Free Mass Index (FFMI). In this article, we will describe what the FFMI is, how it was formulated, and its usefulness.

What is Fat Free Mass Index?

Fat Free Mass Index (FFMI) is an alternative way of looking at health status via general calculations. It was formulated to address some of the concerns that surround the widely used Body Mass Index (BMI) that fails to consider body composition (aka, discriminate between lean mass and fat mass of the body)[1]. BMI fails to address the differences between a muscular individual at a particular weight and a fatter individual at that same weight and height – so, it considers both people at the same level of health, which is inaccurate. FFMI, however, offers an alternative in that it bases its numbers by portioning fat mass and lean mass; this, in effect, creates a more accurate image of muscle on an individual [1].

So, in simplistic terms, FFMI is a calculation like BMI, but with the mindset of separating two major components of body composition (body fat mass and body lean mass) unlike BMI which looks at the two components as one.

How is FFMI formulated?

The calculation for FFMI is the exact same as BMI; however, the subject measured is not total weight, but lean weight [1]. This means that there is an added element of knowing a person’s body fat percentage before being able to use the FFMI and not knowing body fat percent could heavily influence the accuracy of the measure. So, once body fat percent is known, we can figure out fat free mass (lean mass including all muscle) and use the simple “(total kg of FFM)/(total meters in height^2)” to determine our FFMI number.

Is it useful?

There are only a handful of studies that investigate this topic in depth, but of the few that do, there seems to be some use for FFMI. First and foremost, leaner individuals that also pack on sizeable weight via muscle mass, for example, could benefit from this system as BMI would be prone to categorize them as overweight or even potentially obese; which, would lead to a faulty generalization of their health status based on weight alone [2][3].

On the other hand, it is often used as a tool, by bodybuilders, to assess a person’s natural genetic potential in terms of muscle gain. If this is a valid measure is often debated, but since there are no alternatives and it does measure, albeit simplistically, what needs to be measured, it seems FFMI could supply at least some idea of a person’s potential compared to the average.

Natural limit – what is it?

On this last point, a study investigated this exact issue by interview and medical test to determine the difference between natural and steroid using individuals that exercised regularly [4]. Of course, it could be brought up that steroid tests can often be passed by various tricks, but considering not a single person of 157 tested failed their test post interview, it seems highly unlikely these values are off unless we assume 157 people know how to avoid a steroid test without incentive to avoid it. The study also retrospectively investigated 20 Mr. America winners from the years 1939 – 1959 to investigate the very peak of physical accomplishment [4]. We can be 100% positive these individuals (at least the first few) were not on steroids as steroids had not been invented during this time [5]. So, where does that leave us, numerically speaking?

All determined natural lifters (not including Mr. America level) were at or below an FFMI of 25.0 with most being a several points under 25.0 and the genetically “gifted” and knowledgeable sitting around 22.0+ [4]. On the other hand, the average for the steroid users was an FFMI above 25.0, with some extreme cases even entering the 30.0+ category [4]. The mid-ground between steroid lifter and natural lifter, or Mr. America’s – representing the elite natural, also tended to stick around 25.0 with only 3 Mr. America’s assuming an FFMI of over 27.0 – one of which was in 1944 (merely 4-5 years after the invention of steroids for non-competitive purposes)[4][5]. The highest natural FFMI recorded was a 28.0 in 1949 [4].

I will quick mention that the retrospective look at the Mr. Americas needed a good measure of body fat percent to get an accurate FFMI and while the assumption these individuals were extremely lean is likely correct, I do not believe these individuals were 5% body fat, as assumed by the study – therefor, I believe their FFMI to be a bit lower than what the study found. It should also be noted that the measures done on the 157 lifters were done using skin calipers, which have a margin of error; however, if we assume that margin of error to be equal across groups, then the FFMI is likely accurate (or inaccurate) to the same degree.

On the other hand, more recent evidence, looking at FFMI in collegiate athletes seems to be shed a bit more light on the matter, although is not as revealing as others might believe. A study that looked at 235 football players and measured their FFMI using a far more precise body fat measuring system, the DEXA [6]. They showed a range of FFMIs, as one might expect, but also showed an upper limit considerably higher than the Mr. America’s of the previous study; a few football players had FFMIs between 30 – 32 [6]. The average for the linemen was around 25 [6].

Now, this information begs the answer to: “Were they using drugs?” and that is more a judgement call than a fact check, and although the study explains the rigors of the NCAA drug testing policy, I do have my doubts if every player of the 235 from three separate teams was drug free. I say this, because although drugs are expensive, there have been numerous reports, albeit not scientifically evaluated, stating the drug use in collegiate football being more pervasive than one might imagine at first glance. So, is the chance high that a minority of players were using PEDs? I’d say yes, but it is impossible to tease out which players.

So, to set this perfectly clear (according to the research and my opinion):

FFMI of Natural Lifters : Up to 22.0
FFMI of Gifted Natural Lifters: 22.0 – 25.0
FFMI of Elite Natural Lifters (assuming 5%BF on Mr. America subjects): 25.0 – 28.5
FFMI of Steroid Using Lifters: 25.0 and Up

To be clear, these numbers are the majority, but any person in both groups can also be outside of the range by some small margin (for example, one steroid user was as low as 19.0 – this could be due to a variety of factors like lack of experience, time using, lack of nutrition, etc). You will also notice an overlap, and this overlap is unavoidable and only distinguishable via biomedical testing to create that line between natural and steroid use – the FFMI falls short in this regard.

Mr. America 1949 - the only natural studied with an FFMI of 28.0


In summary, we can say that the Fat Free Mass Index (FFMI) is a more precise measure that equates just like the Body Mass Index (BMI), but bases its numbers off fat free mass (aka, muscle, bone, etc). FFMI is useful for distinguishing between truly overweight individuals due to excess body fat and “overweight” individuals due to metabolically active, healthy muscle tissue – so, maybe it should be used in congruence with BMI assuming the body fat measure is accurate and reliable. In terms of its use for determining the “natural” limit of muscle growth, it does provide some useful information, but does not clear up the ambiguous areas between elite level natural lifters and steroid users until the upper limit (FFMI of over 28.0).

Writer: Nicolas Verhoeven

[1] Schutz, Y. (2002). Fat-free mass index and fat mass index percentiles in Caucasians aged 18-98 y. International Journal of Obesity Related Metabolism Disorders, 26(7), 953-960. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/12080449

[2] Kyle, U. G. (2003). Body composition interpretation. Contributions of the fat-free mass index and the body fat mass index. Nutrition, 19(7-8), 597-604. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/12831945

[3] Coin, A., Giannini, S., Minicuci, N., Rinaldi, G., Pedrazzoni, M., Minisola, S., … Sergi, G. (2012). Limb fat-free mass and fat mass reference values by dual-energy X-ray absorptiometry (DEXA) in a 20–80 year-old italian population. Clinical Nutrition, 31(4), 506-511. Retrieved from http://www.clinicalnutritionjournal.com/article/S0261-5614(07)00188-4/pdf

[4] Kouri, E. M., Pope, H. G., Katz, D. L., & Oliva, P. (1995). Fat-Free Mass Index in Users and Nonusers of Anabolic-Androgenic Steroids. Clinical Journal of Sport Medicine, 5(4), 223-228. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/7496846

[5] Anabolic Steroids | CESAR. (2013, October 29). Retrieved from http://www.cesar.umd.edu/cesar/drugs/steroids.asp

[6] Trexler, E. T., Smith-Ryan, A. E., Blue, M. N., Schumacher, R. M., Mayhew, J. L., Mann, J. B., … Mock, M. G. (2017). Fat-Free Mass Index in NCAA Division I and II Collegiate American Football Players. Journal of Strength and Conditioning Research, 31(10), 2719-2727. doi:10.1519/jsc.0000000000001737

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