system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what are muscle "knots"?

When you get a massage, and the therapist works out a "knot", what is that lump in your muscle that they're working out?
gpt
You might not realize it,  but muscles can only pull,  never push. They only contract,  and need to be loosened by an opposing muscle.  So if you're overworked, stressed and don't get enough movement,  some muscles stay contracted,  and need to be loosened up manually. That's why we stretch ourself when we're tired.