In natural individuals, the overall skeletal structure can give you a CLUE about potential muscle mass potential. Wrist size being a fairly decent predictor of potential arm size.
From a sheer size standpoint, a bigger skeletal structure indicates a genetic predisposition to carry a larger amount of overall size.
For upper arms potential, the best evaluation I’ve seen is from Jeff Cavaliere who says that the normal upper arm size potential for most individuals is 10" + wrist size. So with 6" wrists, 16" arms should be achievable by most.
I personally think that there is as much as a 1" variation in individuals who are gifted for muscle growth (1" more in their case) and those who have poor muscle growth genetics (1" less in their case).
However, a smaller bone structure can allow you to give the illusion of more size or gives a more aesthetic physique once well developed (especially when lean) because the smaller joints can make the muscles appear larger or more “tri-dimensional”.
I personally have a small bone structure:
- Small wrists (I think they are 6.5" but I’d have to re-measure them, been years)
- Small hands
- Small head
- Naturally narrow bi-acromial length (shoulder width)
And even though in shirtless/tank top pics I look very big, in reality I’m not. When I’m fully dressed I look “athletic”, that’s it. But when I’m shirtless and lean I give the illusion of having more size. For example my upper arm “looks” big but it really isn’t (by bodybuilding standards) at around 17.5".
CAVEAT: I am referring to my current and “most frequent” physique, which is anywhere between 190 and 205 at low levels of bodyfat. There was a period of my life (that I regret) in which I used PEDs and got much bigger.
Anyway, the point is that a Greyhound will remain a Greyhound regardless of what you do; it will never become a pitbull or a Saint-Bernard.
THAT HAVING BEEN SAID, the worst thing to do is use your structure as an excuse not to progress. Sure, you’ll never have 19-20" arms. But if you do the right things training and nutrition-wise, you should still be able to increase the size of your arms.
The issue is that, as Charles Poliquin pointed out in the past, arm size is correlated with overall body size. You are not going to add 1" to your arms without adding significant weight to your frame, at least not past the beginner bro stage.
Charles used to talk about needing to gain 15lbs of overall mass to add 1" to your arms. I’m not sure if that holds true for everyone but it certainly does apply to people with “poor arm genetics”.
PLUS: I personally hate predicting what is the limit arm size, overall size, chest size, etc. that you can reach. I just find that it leads to a negative and self-defeating mindset (even if it is often subconscious) and that it actually lead you to underachieve what you can reach.
The only things you can control are:
- How hard your train
- How smart you train
- The quality and quantity of your nutrition and supplementation
- How well you are resting
- Stress management
The chips will fall where they will fall. Instead of focusing on what you might possibly achieve, just focus on optimizing the elements above, consistantly.