Same 12-week program. Same exercises, sets and reps, with every session supervised. Eight sites, one protocol, and an MRI of every arm before and after.
Across the 585 people who finished, the change in the trained biceps ran from a 2% loss to a 59% gain. In 232 of them it was between 15 and 25%. But 36 gained less than 5%, and 10 gained more than 40%.
How Much Do Genetics Limit Your Muscle Building Potential?
No paper cited here reports a genetic result, so none can put a number on how much genetics limit you. What a 585-person study does show is how widely people differ on the same program: over 12 weeks, the trained biceps changed by anything from a 2% loss to a 59% gain. Men and women were spread out about equally, and neither age nor training volume predicted who gained most.
— Hubal et al. 2005 · Medicine & Science in Sports & Exercise · n=585
That spread is where any honest answer about genetics has to start. The study was part of FAMuSS, a project set up to find gene variants linked to muscle size and strength, but this paper reports no genetic result. It measured the spread, then tested three possible explanations.
Sex. Men gained a little more on average, 20.4% against 17.9%, which the authors call a slight advantage. But the spread was about the same: the variation in percentage gains did not differ significantly between men and women.
Age. The volunteers were 18 to 40, and age barely tracked size gain (r = −0.09). The authors put that down to the age limit: in most people, they write, testosterone does not fall significantly until 60 or later.
Training volume. Everyone did the same sets and reps, but each person lifted weights set from their own starting strength, so the total weight moved differed. That total showed no relationship with size gain (r = 0.05 in men, −0.09 in women).
So the spread stays unexplained. The paper does not show that genes caused it, and it does not rule them out. What it does show: on identical training, a few people gained little or nothing, a few gained a lot, and 539 of the 585 landed between 5% and 40%.
The study also sorted no one by build, so it says nothing either way about body type categories.
For month-by-month expectations, see monthly gains. And on men and women: across 29 studies, gains relative to starting size were similar. Put both groups’ starting muscle at 100, and at the end men’s would measure about 0.69% more than women’s (the full meta-analysis).