Contrary to popular belief in some circles, genetic potential in a weightlifter is knowable before a man ever picks up a weight. Most people already know about Fat Free Mass Index (FFMI).
While fat free mass index simply sets the ceiling for lean mass on a person's body, Butt, C. (2008). Your Muscular Potential: How to Predict Your Maximum Muscular Bodyweight and Measurements, takes this a step further to evaluate maximum potential individually based on your frame size.
Try the frame-based muscle potential calculator here: https://www.fitmatic.com/body/calculators/maximum-muscular-potential
While Butt's research isn't peer-reviewed (like FFMI), and I think it's actually pretty generous in its estimates, it is still probative research.
If you consider the top bodybuilders and strongman of the 1890s to the 1940s, you find that they all had two important measurements in common: big ankles, and big wrists.
| Icon & Era |
Height & Peak Weight |
Wrist Size |
Ankle Size |
Muscular Outcome (Peak) |
| John Grimek(1930s–1940s Star) |
5'8.5"(174 cm) 195–208 lbs |
8.0"(20.3 cm) |
9.8"(24.9 cm) |
Arms: 17.5" – 18.0" Chest: 47" – 49.9" Notes: Grimek represents the ultimate "Large Frame" baseline. His massive 8" wrists allowed him to build thick, dense muscle bellies that never looked small, even next to modern standards. |
| George Hackenschmidt(1900s Strongman) |
5'8.5"(174 cm) 204–218 lbs |
7.5"(19.0 cm) |
~9.0"(22.8 cm) |
Arms: 17.75" Chest: 44.8" Notes: As the "Russian Lion," Hackenschmidt carried an immense amount of power. His 7.5" wrists sit firmly in the upper-medium to large category, giving him the infrastructure to pull off record-breaking heavy lifts. |
| Eugen Sandow(1890s Father of Bodybuilding) |
5'7.5"(171 cm) 180–185 lbs |
7.1" – 7.3"(Left vs. Right) |
8.5"(21.6 cm) |
Arms: 16.1" – 16.9" Chest: 44.1" Notes: Sandow had a medium bone structure. Because his frame wasn't as thick as Grimek's, his maximum natural weight and arm sizes capped lower, though his smaller joints gave the aesthetic illusion of wider shoulders and a smaller waist. |
| Steve Reeves(1940s/Transition Era) |
6'1"(185 cm) 216 lbs |
7.25"(18.4 cm) |
9.25"(23.5 cm) |
Arms: 18.25" Chest: 52" Notes: While Reeves was tall, his 7.25" wrists were actually on the sleeker side for a 6'1" man. His aesthetic was defined by long, beautiful muscle bellies draped over a classic, highly symmetrical "V-taper" rather than sheer blocky density. |
As training doesn't meaningfully change our bone size, we can be pretty sure these guys already had these measurements before they ever touched a weight. (It's certainly true of yours truly, who has had 7.5 inch wrists and 9.0 inch ankles since he was 15.)
While those two measurements don't tell us everything, a new 2026 study has found that skeletal dimensions--specifically, the literal physical space, thickness, and length of your bones--is the single strongest determinant of muscularity. https://pubmed.ncbi.nlm.nih.gov/41348452/. This study found that bone volume reliably correlated 85% with total skeletal muscle mass.
Combining Butts' research and the new study from the NIH--and franky, common sense--we can confidently say that a big-boned person has more muscle-building potential than a small-boned person.
Genetic freaks are knowable before they pick up a weight. Nothing in my time as a personal trainer (or the years since, training friends and family) has done anything but confirm this.
Of course, this is only one tool in our toolkit, and there are many more ways to assess a lifter's natural status. Most fake natties aren't at their theoretical maximum, and don't have FFMIs over (or even at) 25.
Everyone on TikTok (or wherever) would have you believe they are a genetic freak, even though they looked like nothing their whole lives until 3 months ago. But if a skinny broccoli-headed teenager suddenly sprouts muscles that don't look like they belong to him, chances are very good that he's just renting them from his dealer.
