Scientists Studied 287 People Doing the Same Workout for 6 Months. Why Some Gained 30% Muscle While Others Lost Size (and What You Can Learn From It)

 Ever wonder why some people seem to gain 10 pounds of muscle just by walking into a gym, while others train for months with barely any change?

Turns out, the answer lies in your genetics — and it’s more complex (and more flexible) than you might think.

In this article, you’ll uncover how genetics influence muscle growth rate, which muscles grow fastest, and your maximum potential for size.

You’ll also learn how to work around your “bad genetics” with smart training strategies backed by science — ones used by top natural bodybuilders and researchers alike.

Because while you can’t choose your DNA, you can absolutely hack your way to your best physique.

Genetic Lottery: Why Muscle Growth Isn’t Fair

Some people are born with muscle-building advantages most can only dream of.

Case in point: Julian, a teenager whose physique was so eye-catching that fitness expert Jeff Nippard dedicated an entire video to investigate if he was natural.

His arms? Comparable in size to the average adult’s leg.

Then there’s Paul Durand, another standout with what may be a rare myostatin gene mutation—this mutation limits muscle growth in most people, but not in him.

“I noticed I was different around 5 or 6,” said Paul. “Even before weight training, I had definition and size most adults didn’t.”

What is Myostatin — and Why Do You Have It?

Myostatin is a protein that literally stops your muscles from growing too large.

From an evolutionary standpoint, it helped our ancestors conserve energy, as muscle burns a lot of calories at rest.

But in today’s world, it’s a growth-limiting barrier — unless you’re born with reduced myostatin levels.

In rare cases, humans with mutations in the myostatin gene are “super responders,” developing large muscles with minimal training or diet precision.

Still, these mutations are extremely rare, affecting around 4 in a million.

Muscle Growth Speed: The 3 Types of Responders

If you put 287 people on the exact same strength training program for 6 months, their results won’t be equal.

This happened in a 2016 study where researchers used MRI, DEXA, and ultrasound imaging to assess gains.

Here’s what they found:

• Hyper Responders: Gained up to 30% muscle, expanding two to three times more than average.
• Average Responders: Most participants, with 5–10% muscle increases.
• Low Responders: Gained very little or even lost muscle despite consistent training.

And remember, this was with identical training and diet protocols.

Your rate of growth may not be your fault. But you can still change your outcome.

How to Know if Your Genetics Are “Good” or “Bad”

Looking broad-shouldered or stocky can give the illusion of good genetics.

In one recent study, lifters with wider frames did start off with slightly more muscle, but they didn’t grow much faster.

The most reliable indicator? Your gains in muscle and strength during your first year of training.

If your lifts jump quickly and your physique changes dramatically, you probably have above-average genetics.

Working Around Genetics with Smart Training

Strategy #1: Increase Your Training Volume

Pro bodybuilder and research scientist Dr. Eric Helms shared his breakthrough:

“After switching to higher training volume, I saw the biggest improvements in 20 years of lifting.”

Volume = Number of hard sets you do per muscle group per week.

If you’re under 12 sets per muscle per week, a 20–30% bump can lead to major growth, especially for low responders.

• Under 12 sets/week: Add 2–4 more sets
• 12–20 sets/week: Go higher only if your form and recovery are optimal
• Over 20 sets/week: Gains plateau or reverse — more isn’t better here

Strategy #2: Specialization Training

If you can’t gain size everywhere at once, focus on where it matters most visually.

For men, that’s typically shoulders or arms. For women, it’s often glutes.

To specialize:

• Pick 1–2 lagging muscles
• Start at 10 sets/week
• Gradually bump up to 20–30 sets/week based on recovery
• Maintain other muscle groups with just 8–12 sets/week
• Drop naturally strong areas to 4–6 sets/week

This “volume budget” ensures you’re using your recovery capacity where it matters most.

That’s how the author fixed his narrow frame by building 3D shoulders — and how his wife built glutes that outgrew her supposed genetic ceiling.

Your Natural Limit: FFMI

Fat-Free Mass Index (FFMI) is a scientifically accepted way to estimate your muscle-building ceiling.

Here’s a breakdown of average scores:

• 18–19: Beginner or untrained
• 20–21: Moderately lean, athletic (Bruce Lee level)
• 22–23: Advanced lifter — top 5% of the general population
• 24–25: Elite natural genetic outlier
• 28–30: Steroid enhanced territory (The Rock, Cena)

To calculate your FFMI easily, use the Built with Science Calculator.

What Your Genetic Ceiling Looks Like

Based on decades of data compiled by Dr. Eric Trexler:

• Low Responders: Peak at ~FFMI 20–21 (~20 lbs muscle gain)
• Average Responders: Up to ~22–23 (~30 lbs lifetime gains)
• High Responders: FFMI 24–25 (~40+ lbs muscle gains)

Even low responders can build a strong, lean look better than 70% of the population.

The “Secret” Most People Miss: Training Right

Zach Robinson, a muscle growth researcher, found that much of the difference in study results wasn’t genetic.

It came down to things you can control:

• Nutrition
• Effort in training
• Sleep and recovery
• Consistency

One client, Brian, thought he was a hardgainer for years — until the program adjustments revealed he actually had elite potential.

“He wasn’t under-achieving because of genetics — it was his plan that was failing him,” the author explains.

Final Thoughts: Your Genes Are the Starting Line, Not the Finish

Whether you’re a high responder or someone who struggles to gain a single inch on your arms, know this — you’re not stuck.

By training smarter, specializing your effort, and dialing in your lifestyle, you can transform your physique beyond what you thought possible.

Want to fast-track your plan? Try the Built with Science Plus app for free. It automatically adjusts your workout to target weak spots based on your real-world results.

Not sure where to begin? Start by watching this shoulder specialization video that helped create a broader, more aesthetic frame — no matter your bone structure.

Because great genetics help, but great effort always wins over time.