system
Given the following post, respond as if you're commenting on Reddit:
human
Does the brain send signals consistently to keep a muscle in the same state?

When I, for example, hold one arm straight to the side, does the brain continuously give the signal to keep the muscle extended or does it just make the arm extend once and it will stay extended until a different signal comes in?
gpt
yes - you can think of muscles as huge bundles of motor units, which are small bundles of muscle cells.  Each motor unit has its own motor neuron.  When you raise one arm out to the side and are holding it there, your motor cortex is sending out a consistent stream of signals to keep that arm up.  However, it requires more force to raise that arm to its holding position, and therefore, requires recruitment of a greater number of motor units (more signals sent from the brain) to initially raise the arm.

It is actually pretty interesting how it works.  Motor units can't stay contracted for very long, or else once the arm was raised the signals would stop.  Instead, as individual motor units run out of ATP (the main cellular form of energy), they relax and the force that they were generating to keep the arm up is replaced by another motor unit that was previously relaxed.

To simplify - at the beginning of a muscle contraction, there are a really really large amount of individual signals, the number of signals then decreases but remains consistent until you lower the arm.