system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does building muscle work? Why does using your muscles result in larger more developed ones?
gpt
**What are they?**
Muscles are made of cells, like the rest of your body, but these cells are special. Skeletal muscle cells in particular are kind of long and stringy, and these strings can grow in pretty much any direction, and in multiple directions. Picture an octopus or something. These are what make a muscle do work - electrical signals from the nervous system can activate a chemical reaction, causing the muscle fibers to contract, with force, placing active muscle fibers and everything they pull against under stress. This causes damage to the cell membranes - if the damage is minor, it can simply heal itself, but...

Cells patch larger tears in their membranes by growing new fibers out of the site. The end of a newly grown fiber is "sticky," causing the fiber to attach to any other muscle cells it may grow in to. If the fiber remains attached for long enough, it will permanently integrate itself into the cell it's attached to. As a result, multiple cell nuclei end up being enclosed by the same membrane - so it's hard to say where one cell ends and another begins.

All this growth requires protein and other nutrition.

**Maybe start here**

When you work out by lifting heavy weights, you put a lot of stress on your muscles, and actually end up doing quite a bit of damage to them. They take a few days to patch the widespread tearing with newly grown muscle fibers. These fibers grow in all different directions. Skeletal muscles can only do work in one direction, though - so fibers growing "against the grain" contribute to muscle mass but add nothing to muscle strength.

Wait, there's more!

Keep in mind that the "sticky end" of a new fiber becomes more strongly anchored over time - until then, it's far easier to "unstick" it than it is to damage it. This is what causes the fibers to eventually grow in the same direction - pulling (and widening) at an angle to other actively contracting fibers puts far more stress on a single strand compared to contracting in parallel.

However, this "migration" process takes much longer than simply growing more muscle fibers. Some fibers attach more strongly or migrate more quickly than others.

Poorly positioned fibers, if allowed to anchor permanently, greatly (and permanently) increase the occurrence of additional "cross grain" fibers forming after the next workout, especially on the outer surface of the bundle. Without invoking a mathematical explanation, let's just say it's messy.

The two cycles are different lengths. Repair and growth are complete after a couple of days. Migration is more of an optimization process, and may take weeks or even months to approach 100%. This is a big part of why lifting heavy weights as often as possible usually results in big muscles with poor performance and no definition, and why mass gain drops off over time, and why a point of no return exists where definition is concerned.