The Study
A systematic review and meta-analysis examining if hyperplasia occurs in humans in response to resistance exercise.
This study looked at lots of other studies to see if lifting weights makes new muscle fibers grow in people. It found that most of the time, muscles get bigger by making existing fibers larger (hypertrophy), not by adding new ones. But because we can't count muscle fibers exactly in living people, we can't say for sure that no new fibers ever form.
Analysis score
Maximum 100 for a systematic review with meta-analysis.
Where the score came from
When people lift weights, their muscles get bigger, but this study checks if that's because they're adding new muscle fibers or just making existing ones bigger.
Where does this study sit?
Reviews of RCTs (Meta-analyses)
Max 100Randomized Trials
Max 90Reviews of Cohort Studies
Max 85Cohort Studies
Max 72Reviews of Case-Control Studies
Max 63Case-Control Studies
Max 58Cross-Sectional & Case Series
Max 50Expert Opinion
Max 539 / 100
Quality score
The highest quality evidence. Systematic reviews and meta-analyses that pool randomized controlled trials, giving the most reliable summary of experimental evidence.
Key takeaways
Summary
Based on the study abstract and findings.
- 1The change isn’t big enough to matter in real life, and it looks like muscle growth comes from fibers getting thicker, not from adding new ones.
- 2On average, there was a change of 4,086 fibers, but the range was huge—from losing 6,430 to gaining 14,604 fibers—so we can't say for sure there was any real change.
Score breakdown, methodology, conflicts of interest, evidence analysis & raw study data
Publication
Journal
The Journal of sports medicine and physical fitness
Year
2025
Authors
Nicholas V. Barton, Hitesh N. Gowda, Scott J. Dankel
Related Content
Claims (6)
When you lift weights, your muscles get bigger not because you're growing new muscle fibers, but because the ones you already have are getting thicker. Scientists don't think you actually create new muscle fibers from training.
Whether you're new to lifting weights or already experienced, your muscles seem to add new fibers at about the same rate when you start resistance training — at least in the first 6 months.
Lifting weights for up to 6 months probably doesn’t increase the number of muscle fibers in your arms or legs — instead, your muscles grow because the existing fibers get bigger, not because you grow new ones.
How long you lift weights—up to 6 months—doesn’t seem to change the number of muscle fibers you have, according to this claim.
When people lift weights, their arm and leg muscles show similar changes in muscle fiber numbers — and in both, we don’t see clear signs of new muscle fibers forming.
Right now, the way scientists guess how many muscle fibers someone has isn't very precise — the numbers jump around a lot, so it's hard to spot small changes in muscle fibers.
Not medical advice. For informational purposes only. Always consult a qualified healthcare professional before making health decisions.