system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does your neck get tense when you’re stressed and how does a massage fix it?
gpt
When you are stressed, you experience fight-or-flight, even if the stressor is emotional/psychological in nature.  When this response occurs, it causes you to produce more adrenaline.  Adrenaline causes muscle tension (muscle tension being a sign of stress).  If you massage a muscle, you can cause that muscle to relax.  So stress increases adrenaline, causing muscle tension, massage relaxes muscles.