The difference between how fast men and women build muscle is 0.69%. Twenty-nine studies. 2,815 measurements. The meta-analysis couldn't even say with confidence which sex grows faster.
“The fear of getting bulky from lifting weights is the equivalent of worrying that your savings account will grow too fast — when it has less money in it than the person's next to you.”
The difference between how fast men and women build muscle is 0.69%.
Twenty-nine studies. 2,815 measurements. Men and women doing the same resistance training programs, measured with every tool available — MRI scans, ultrasound, muscle biopsies, body composition scans. When researchers pooled all of it into a single analysis in 2025, the gap in relative muscle growth between sexes was less than a single percentage point.
Not 10%. Not 5%. Zero point six nine percent. The meta-analysis couldn't even say with confidence which sex grows muscle faster — there's a 26% probability that women actually build it at a slightly higher rate than men. The finding was so close to zero that the question itself stops making sense.
So if the rates are identical, why do men end up with more muscle?
Women and men build muscle at virtually the same rate. The entire difference across 29 studies is a fraction of a percent — and the meta-analysis couldn't even determine which sex grows faster.
- Men end up with more muscle because they start with more — not because they grow it faster. Same growth rate, different starting size.
- The equal-rate finding held across every body region, every measurement method, and both trained and untrained participants.
- The study found that resistance training programs can follow the same structure for both sexes — differences in programming should come from individual goals, not from sex.
- One fibre type (Type I) may grow differently between sexes — but the evidence comes from just seven studies with wide uncertainty, and the researchers flag it as inconclusive.
- An earlier review using completely different statistical methods reached the same conclusion — two independent research approaches, same answer.
Same Interest Rate, Different Starting Balance
Think of muscle like a savings account.
Two people open accounts at the same bank. One deposits $10,000. The other deposits $5,000. Both accounts earn 5% interest. After a year, the first account holds $10,500 and the second holds $5,250. The first person didn't earn interest faster — they started with a bigger deposit.
That's the entire sex difference in muscle growth. Men start with more muscle mass. In the studies Refalo's team analysed, average male muscle fibre cross-sectional area was 4,616 μm² compared to 3,652 μm² in women — roughly 26% more tissue at baseline. Apply the same growth rate to a larger starting mass and you get a larger absolute increase. The rate never changed. The starting balance did.
The meta-analysis confirmed this with a precision that's hard to argue with. For absolute growth — the raw amount of tissue gained — men came out slightly ahead, and the evidence was unanimous.
But for relative growth — the percentage gain from your own starting point — the difference collapsed to 0.69%, with only 74% probability that the difference even pointed in men's direction. Same interest rate. Different opening deposit. The account with more money in it didn't grow faster. It just had more money to begin with.
Which means the fear of "getting bulky" from lifting weights is the equivalent of worrying that your savings account will grow too fast — when it has less money in it than the person's next to you.
The meta-analysis pooled 29 studies and still couldn't say with confidence which sex grows muscle faster. There's a 26% probability that women actually build it at a slightly higher rate — the difference is so small it's essentially unresolvable.
The Word That Doesn't Exist
If men and women build muscle at the same relative rate, why does the fitness industry sell them completely different programs?
Walk into any commercial gym and you'll find two vocabularies. On one side: bulk, gain, mass, grow. On the other: tone, sculpt, lengthen, lean. The exercises are often identical. The rep ranges overlap. The progression model is the same. But one version comes in a different package, uses different language, and frequently costs the same or more.
"Toning" is not a physiological process. The word has no definition in exercise science. What people mean when they say "toned" is a specific visual result: moderate muscle mass visible under relatively low body fat. That's mild hypertrophy plus fat loss. It's the same adaptation men call "getting lean."
Someone invented a separate word for the same biological process. Made it sound less intimidating. Built an entire product category around it.
Refalo's meta-analysis found that resistance training experience didn't change the picture. The equal growth rate held across both untrained people and those who had been lifting for years.
And the paper's own conclusion states directly that programs "can follow similar structures for both untrained and resistance-trained males and females." Not similar programs adjusted for sex. Similar structures. Full stop.
The gendered version of strength training is a marketing construct. Not a biological requirement.
“Toning is not a physiological process. The word has no definition in exercise science. Someone invented a separate word to describe the same biological process, made it sound less intimidating, and built an entire product category around it.”
The Honest Edge
There is one corner of the data where a sex difference might actually exist.
When the researchers zoomed in on muscle fibre types, Type I (slow-twitch) fibres showed a possible advantage for men. The probability of greater absolute and relative growth favouring males was 90% or higher. But this came from just seven studies with 170 total participants, and the uncertainty ranges were wide enough to include the possibility of no difference at all.
Type II (fast-twitch) fibres told a different story. The meta-analysis estimated a negligible difference that actually tilted slightly toward women. The relative change was −2.29%, with only 62% probability in that direction.
The researchers themselves flagged the variability in how different labs measure fibre size. Some studies counted as few as 37 fibres per participant. Others counted 374. That range alone is reason to interpret fibre-type findings with caution.
This matters because honesty about uncertainty is what separates reliable analysis from marketing. The main finding — 0.69% difference across 29 studies — is built on a foundation of 2,815 data points with virtually no disagreement between studies. The fibre-type finding is built on 170 participants with wide uncertainty ranges.
Both are real findings from the same meta-analysis. But one is a skyscraper and the other is scaffolding. Knowing the difference is how you avoid overclaiming in either direction.
“Your muscles grow at the same rate as everyone else's. The 0.69% that separates you from the person across the gym is a rounding error spread across 29 studies and 2,815 measurements.”
The Wall of Evidence
What makes the 0.69% finding particularly hard to dismiss is how little it moved no matter how the researchers sliced the data.
Upper body vs lower body? Similar relative growth in both regions — 0.60% and 1.04% differences, neither large enough to matter. Muscle thickness vs cross-sectional area vs volume? Similar across every measurement method. Trained vs untrained participants? 0.87% and 0.66% — virtually identical, and both negligible.
The 29 studies agreed with each other to a degree that's unusual in exercise science — virtually no variation between them. When the researchers stress-tested the results under different assumptions, the answer barely moved. The finding wasn't fragile. It was reinforced from every angle.
This isn't one study making a bold claim. It's a meta-analysis where every sub-analysis pointed in the same direction: no meaningful sex difference in relative muscle growth.
An earlier systematic review by Roberts, Nuckols, and Krieger (2020) used completely different statistical methods. It reached the same conclusion across 10 studies — no meaningful difference in muscle growth, with near-perfect agreement across every study it pooled. Two different research teams, two different statistical frameworks, the same answer.
What Changed
The numbers haven't changed. The weight room looks exactly the same as it did yesterday. The dumbbells didn't get lighter. The barbells didn't rearrange themselves.
What changed is what you know about your own body. The fear was real — years of messaging, transformation photos taken out of context, a fitness vocabulary built on the assumption that women's muscles work differently. That fear shaped real decisions, real routines, real avoidance of equipment that could have been serving you the entire time.
But the math was always on your side. Your muscles grow at the same rate as everyone else's. The 0.69% that separates you from the person across the gym is a rounding error spread across 29 studies and 2,815 measurements. The boundary you felt at the dumbbell rack was never biological. It was inherited.
Now you know. And the next question — whether you even need heavy weights, or if lighter ones do the same job — has an answer too.
The growth rate question is settled. What changes is how you think about program design.
Women may actually tolerate more training volume than men. Research outside this meta-analysis suggests women are less fatigable during resistance training — they can perform more reps at a given percentage of their max and may recover faster between sessions. The industry sold women gentler programs when the data points in the opposite direction.
The only variables that should drive your program are individual — your goals, your recovery capacity, your preferences, your schedule. Not your sex. The paper's own authors say programs can follow similar structures for both sexes, with differences based on long-term objectives and personal factors like enjoyment and stress tolerance.
What other research found
What this means for you
The finding is actually strongest for beginners. Twenty-three of the 29 studies tested people with no resistance training experience, and the relative growth difference was 0.66% — even smaller than the overall finding.
Your muscles don't know you're new. They grow at the same percentage rate as everyone else's from the moment you start. The fear that beginners somehow bulk faster has no support in this data.
Six studies in this meta-analysis specifically tested people with resistance training experience. The relative growth difference for trained participants was 0.87% — still negligible, still within the range where the meta-analysis couldn't determine direction.
The paper states directly that programs can follow similar structures for both sexes. If your current program was designed around the idea that women's muscles respond differently — the data says otherwise.
The programming implication is clear: the hypertrophy stimulus doesn't need to differ by sex. The paper's practical applications section names individual factors — goals, enjoyment, discomfort tolerance, stress capacity — as the basis for program variation.
Research also suggests women may be less fatigable during resistance training and may recover faster between sessions. That means female clients might handle higher training frequency or volume — the opposite of what gendered programming typically prescribes.
Before you change anything
Healthy adults aged 18 to 45 — that's who this meta-analysis studied. If you fall outside that window, the findings may not apply the same way.
A separate meta-analysis (Jones et al. 2021, cited in this paper) looked specifically at adults over 50 and found similar relative growth between sexes — but this paper's data doesn't include them. People taking anabolic steroids, those with medical conditions affecting muscle growth, and adolescents were all excluded.
The finding is strongest for the general gym-going population — healthy adults of typical training age. That's a broad group, but it's not everyone.
Most participants were beginners. Twenty-three of 29 studies tested untrained people. Only six included anyone with resistance training experience — and even then, the exact training history was often vaguely described.
Studies were short. Training periods ranged from 6 to 24 weeks, with an average of just 11 weeks. The growth rates are equal over that timeframe. Whether they diverge over years of consistent training is a question this data can't fully answer.
Program designs varied across studies — different exercises, different rep ranges, different frequencies. The low between-study variance suggests this didn't matter much, but it's worth noting.
For the main finding — equal relative growth rates — confidence is high. Twenty-nine studies, 2,815 data points, negligible disagreement between studies, and an independent review using different methods reached the same conclusion.
For trained populations specifically, confidence is moderate. Six studies showed the same pattern, but the evidence base is thinner.
For fibre-type differences, confidence is low. Seven studies with 170 participants and wide uncertainty ranges. The researchers themselves say these findings need more investigation.
The growth rate question is answered — your muscles build at the same pace regardless of sex. But that still leaves the weight room's other unspoken rule: the assumption that building muscle requires heavy weights.
A 21-study meta-analysis tested that directly, comparing light and heavy loads across 684 people. The answer dismantles the second barrier between you and the dumbbell rack.
What This Study Found
All findings from this paper, in plain language.
- Men gained slightly more total muscle than women — but the difference was small enough to be considered negligible.
- When measured as a percentage of their starting size, men and women built muscle at virtually the same rate — a difference of just 0.69%.
- The reason men gain more total muscle is that they start with more — not because their muscles grow faster.
- The equal growth rate held for both upper and lower body muscles, though the absolute gap was slightly wider in the upper body.
- Slow-twitch muscle fibres may grow differently between sexes, but only seven studies measured this and the results were uncertain.
- Fast-twitch muscle fibres showed no meaningful growth difference between men and women — if anything, women had a slight edge.
- Whether participants were experienced lifters or complete beginners didn't change the equal-rate picture.
- The finding held regardless of how muscle was measured — MRI, ultrasound, biopsy, or body scans.
- The 29 studies agreed with each other to an unusual degree — very little variation in results between them.
- Testing the math under different assumptions barely changed the answer — the finding is robust.
- Training programs don't need to be designed differently for men and women — the same structure works for both.
- There was no sign of publication bias — studies showing equal growth weren't selectively published.