Editing Openai/696eca7b-acdc-800a-9ad9-5ebc9f477576 (section)

=== Assistant: Short answer: both happen, but mostly the first—and DNA amount doesn’t change. ===
Short answer: both happen, but mostly the first—and DNA amount doesn’t change.

Here’s the precise breakdown.

===== For skeletal muscle (the kind you build in the gym), increased muscle mass comes primarily from: =====

Hypertrophy = existing muscle fibers (cells) get larger
* Each muscle fiber is already a very large multinucleated cell.
* Training causes more contractile proteins (actin & myosin) to be synthesized → the fiber thickens.

Hyperplasia = new muscle fibers form
* This can occur in some animals and possibly in humans under extreme conditions, but in humans it’s considered minimal and controversial compared to hypertrophy.

So when people say “more muscle,” they usually mean bigger fibers, not more fibers.

===== Your question gets more interesting here. =====

Muscle fibers are multinucleated cells. When they grow, they recruit satellite cells (muscle stem cells) that fuse to the fiber and donate extra nuclei.

So:
* Each nucleus still contains the same DNA (same genome).
* But the number of nuclei per fiber increases.
* That increases the fiber’s capacity to produce RNA and protein, which supports the larger size.

This is called the myonuclear domain theory.

So technically:
* Total DNA content per muscle fiber increases (because more nuclei are added),
* But DNA per nucleus does not change.

===== No — the sequence doesn’t change. =====

But what does change is:
* Gene expression (epigenetics, chromatin accessibility, transcription rates)
* This is why trained muscle behaves differently even after detraining (sometimes called muscle memory at the cellular level).

===== - More muscle = mostly bigger cells (hypertrophy) =====
* Some increase in nuclei via satellite cell fusion
* No change in DNA sequence
* More total DNA per fiber because there are more nuclei
* Long-term training changes gene expression patterns, not genes

If you want, I can also explain:
* The limits of hypertrophy
* Why muscle memory persists
* How this differs in heart muscle vs smooth muscle
* How this compares to fat tissue expansion