system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How exactly do muscle cramps/spasms (for example, calf cramps) occur? What is the process that causes them?
gpt
Your muscles move by contracting, and then they expand again when at rest. When you have a cramp, the muscles have not been able to expand for whatever reason, so they remain tensed and contracted which can become painful. 

One reason for this can be a lack of electrolytes. The muscles use specific ions like calcium and sodium to contract and expand, and if you don't have enough of the right ion the muscles won't be able to expand after contracting. This is why Gatorade has electrolytes mixed in with it. A lack of water will do the same thing. 

Another reason could be the nerves sending the signal to your muscles have gotten confused, sending the signal to tense over and over again. 

It could even be part of your brain mistakenly telling your nerves to send a signal to tense over and over.