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.

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.

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.

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.

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.