system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When people say they can feel knots in your back, what exactly are those knots made of?

Are those knots stress? How do they form exactly?
gpt
After a muscle experiences an extended period of strain or some awkward use, some of the fibrils in a band can remain contracted or clenched, even though the muscle is signalled to relax.

Imagine a thick rope, being made up of several smaller strands all tied together.  Now imagine if a bunch of those smaller strands shrunk, while the rest remained loose.  That's a knot in your back.