Women and Muscle Building: What the 2025 Science Actually Says

If you've spent any time in fitness spaces, you've likely heard claims that men and women build muscle differently. Marketing tells us women need "toning" programs while men need "building" programs. Gym culture often suggests women should use lighter weights and higher reps.

But what does the actual science say?

A groundbreaking 2025 meta-analysis by Refalo and colleagues has finally provided some definitive answers—and the results might surprise you.

The Big Finding: Relative Gains Are Nearly Identical

The 2025 meta-analysis, published in PeerJ, examined nearly 30 studies where men and women performed identical training programs. The key insight: when you control for starting muscle mass, men and women gain muscle at virtually the same rate.

Here's what that means in practice:

• Men typically start with more muscle mass
• Men typically gain more absolute muscle (total pounds/kilos)
• But the percentage increase? Nearly identical

A woman starting with 20 lbs of muscle who gains 10% has added 2 lbs of muscle. A man starting with 30 lbs of muscle who gains 10% has added 3 lbs. The man gained more total mass, but the response to training was identical.

This finding held across different assessment methods (ultrasound, DXA, MRI) and across all experience levels—from beginners to advanced lifters.

What About Strength?

When it comes to strength gains, the data gets even more interesting. A 2020 meta-analysis by Roberts et al. found that:

• Women showed greater relative increases in upper-body strength than men
• Women showed comparable relative increases in lower-body strength

This challenges the old narrative that men are "universally more responsive" to training. Women often start further from their genetic ceiling, so early adaptations can actually appear more dramatic on a percentage basis.

For older adults, a 2021 meta-analysis found that while men gained more absolute size and strength, women once again matched or exceeded them on a relative basis—particularly in lower-body strength. This is especially relevant for the Jacked app's focus on functional fitness and longevity.

The Fiber Type Story

One nuanced finding from the 2025 research: men experienced slightly greater increases in Type I (slow-twitch) fiber cross-sectional area, while Type II (fast-twitch) fiber hypertrophy was roughly equal between sexes.

This matters because:

• Type I fibers are endurance-oriented
• Type II fibers are power and strength-oriented

The implication? Both sexes can expect similar gains in the "power" side of things, which is what most people care about for muscle building.

What This Means For Your Training

Here's the practical takeaway: there's no evidence that women need fundamentally different programs than men for muscle building. The same principles apply:

• Progressive overload — Continually challenge your muscles
• Train close to failure — Get within 1-3 reps of failure for optimal growth
• Eat enough protein — Aim for 0.7-1g per pound of body weight
• Recover adequately — Sleep, stress management, and rest between sessions

The "women should use light weights for toning" myth? Debunked. Women can—and should—lift heavy to maximize muscle growth, just like men.

The Real Differences (That Don't Matter Much)

Yes, some biological differences exist:

• Men typically have more total testosterone (though women's testosterone still supports muscle growth)
• Men typically have more muscle mass to start with (largely due to puberty)
• Hormonal fluctuations across the menstrual cycle can cause small performance variations

But these differences don't require separate programming strategies for most women. Instead, program design should be based on:

• Individual goals
• Training experience
• Recovery capacity
• Personal preferences

Conclusion

The narrative that women have a smaller response to resistance training doesn't hold up to scientific scrutiny. The 2025 meta-analysis confirms what many in the field suspected: when you look at relative gains, men and women build muscle at remarkably similar rates.

So whether you're a woman following the Jacked app's autoprogression program or a man doing the same workouts—the science says you're both on equal footing when it comes to building muscle. Lift heavy, eat right, and trust the process.

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References:

• Refalo et al. (2025). Sex differences in absolute and relative changes in muscle size following resistance training. PeerJ
• Roberts et al. (2020). Sex differences in resistance training. Journal of Strength and Conditioning Research
• 2021 meta-analysis on older adults. Sports Medicine