system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Why do repetitive movements cause inflammation (ex tendonitis)? What exactly causes the muscle to become inflamed?
gpt
it's not the muscles, it's the tendons.  Hence, tendonitis.  

Repeatedly flexing tendons do not make them stronger.  They don't benefit from exercise.  In fact, repetitive actions weaken them since they're not made of muscle.  They're made of collagen, and while they're naturally resistant to damage, repeated actions over long periods of time can overcome this and cause real damage.