system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is different about my butt muscle that stops it from cramping!

If I tense any muscle in my body for too long, it loses strength, or cramps. But my butt muscle (anus, sphincter - whatever - not my glutes) is "tight" and tensed almost all day. What is so special about it that allows it to remain tense all day without cramping or relaxing?  Is there anything about it's abilities that could be used to allow other muscles in the body to work longer/harder?
gpt
Believe it or not, your external anal sphincter that you can voluntarily control (skeletal muscle) isn't contracted all day. It is in fact relaxed most of the day. Your internal anal sphincter is contracted all day. When the stretch receptors in your rectum are activated by feces, your involuntarily controlled internal anal sphincter relaxes and you have to voluntarily contract your external anal sphincter in order to prevent defecation. 

TL:DR You only use your butthole muscle when you have to actually take a shit. 

Btw, The internal anal sphincter is made up of smooth muscle, which works a little differently than skeletal muscle and can work all day without cramping.