Even though muscles get bigger after 10 weeks of weightlifting, the individual muscle fibers don’t necessarily get thicker — the growth might come from how the fibers are arranged, not from each one swelling up.

Scientific Claim

Muscle fiber cross-sectional area does not significantly increase on average after 10 weeks of resistance training to failure at 60% 1RM, despite measurable whole-muscle hypertrophy, suggesting structural adaptations like pennation angle or sarcomere addition may contribute more than fiber thickening.

Original Statement

“Training did not increase the average fiber CSA measured from muscle biopsies in either type I or type II fibers in either typology group or frequency condition (p > 0.05).”

From study:Can muscle typology explain the inter‐individual variability in resistance training adaptations?

Evidence Quality Assessment

Claim Status

appropriately stated

Study Design Support

Design supports claim

Appropriate Language Strength

definitive

Can make definitive causal claims

Assessment Explanation

The study used paired muscle biopsies and MRI to directly compare fiber CSA and whole-muscle volume changes, finding no significant fiber CSA increase despite clear hypertrophy — a robust, direct comparison supporting definitive description.

Evidence from Studies

Supporting (1)

Can muscle typology explain the inter‐individual variability in resistance training adaptations?

Randomized Controlled Trial

Human

2023 Jun

Even though people’s muscles got bigger after 10 weeks of lifting, their individual muscle fibers didn’t consistently get thicker—so something else, like how fibers are arranged, probably caused the growth.

Contradicting (0)

No contradicting evidence found

Source Study

Can muscle typology explain the inter‐individual variability in resistance training adaptations?

Score: 62

DOI: 10.1113/JP284442

Similar Assertions

Even though muscles got bigger, the actual muscle fibers didn’t get thicker or change type—so the growth might be due to other factors like fluid or connective tissue.

82%

The slow-twitch muscle fibers got bigger no matter what kind of workout or drink the men used—whether they lifted slowly up or down, or drank protein or sugar water.

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There are only a few small, short studies on this topic, and many didn’t even track whether people stuck to their workout plans — so we can’t trust the results very much.

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We just don’t know yet if light or heavy weights make muscles grow better — the results could swing either way in future studies, so we need more research.

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Whether you lift light weights or heavy weights until you can't do another rep, your slow-twitch muscle fibers in your thighs grow about the same amount — but we can't be sure because the results are too uncertain.

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