Let us return to Anatomy 101.
Humans have three types of muscle. First, of course, is skeletal muscle - the ones you see at the surface, your biceps, your quads, your glutes. Secondly is cardiac muscle that's at the heart of it all. And then there's smooth muscle, which is the muscle that makes up your GI tract, reproductive tract, and the uterus in women. You can also find it some other obscure places (your iris, bile duct, your blood vessels, etc.)
It's called smooth because it doesn't have the striations that skeletal muscle has, on account of its fibers being a lot smaller. That's good news for you, because being so smooth makes it pretty elastic, which means you don't die when you eat too much or get a headache. Unfortunately, that also makes it weaker and more susceptible to strain and tearing, and its growth is primarily tied to hormones, so you can expect a long recovery.
For the most part, the smooth muscles in your body run involuntarily. When they stop, you've got issues: atherosclerosis, lupus, cirrhosis. Good luck with that.
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