system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do muscles start to twitch for no apparent reason?
gpt
Probably a misfire within a nerve bundle that triggers a motor unit in a muscle.

Muscles aren't controlled the way you probably think they are. Your muscles are roughly divided into sections called motor units, each fed by a nerve (nerves are all or nothing, either completely on or completely off, so they can't be used in isolation for gradients, making parallelism the only viable strategy for precise motor control). 

Whenever your brain wants to send a command to your muscles, it goes through a process called recruitment, where it gathers up the required number of motor units to carry out whatever activity it's doing (more units in a line means larger motion, more in parallel means more force), and then sends the signal to all of the motor units in whatever pattern is needed (maybe series contraction first, then more in parallel, sort of a soft start). 

Recruitment is carried out mainly by the cerebellum in activities you have practiced, because it is more efficient, but less adaptable on the fly. When you are learning a new task, your cortex takes on recruitment, which is why it's clumsier, less precise, and slower, because it can do basically anything, but not very well. This is also why it is harder to do anything other than the new task at first, because your cortex is already in use. Device not ready bro. If your brain misfires slightly, then recruits and fires a single unit, that one little muscle spot twitches.

It has been postulated that we aren't as strong as similarly sized mammals because our muscles are divided up more finely, requiring more brainpower and time to recruit. This allows us to carry out extremely precise actions (flint knapping, for instance), but makes it more difficult to use our entire muscle at once (there just aren't enough connections and they can't be recruited fast enough).

Working out and doing complex physical tasks helps strengthen the recruitment mechanism, which is thought to be the main driving force behind "beginner gains" in bodybuilding. Your brain just learns how to use your muscles better.

Edit: Gold?!? Noice. Thanks kind internet stranger!

Edit: Initial statement for clarity.