system
Given the following post, respond as if you're commenting on Reddit:
human
Why do muscles shake when we tense them?
gpt
Your muscles are innervated by motor neurons - nerve cells that signal to the muscle to contract. To activate a muscle, the central nervous system sends a signal via the motor neurons and the motor neurons "fire" on the muscle with pulses.

There are generally speaking three types of motor neurons which are classified according to how easily they fatigue. These are sometimes nicknamed as fast or slow twitch. 

Now, even the fastest of these neurons only fire at around 60hz (60 "shots fired" per second), but the slower ones fire even slower, around 20hz per second. 

The result of this is that muscle firing is that the movements (or in this case, flexing/tensing the muscles) is not a static thing but made up of fast "bursts" which cause this visible "shaking" to occur.

Now the more a person moves with the slower twitch fibers, the more obvious this shaking will be.

Does this answer your question?