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I don’t know about you, but I consider any day I don’t poop my pants to be a good day. When you think about it, though, it's kind of weird that we’re able to do that.

The muscle that keeps things from leaking out of your body, your anal sphincter, is always clenching—. So why doesn’t it get tired like any other muscle if you always clenched it? Turns out, your sphincter isn’t super ultra strong or anything, but it is more energy efficient.

You have dozens of sphincters in your body, including the one in your anus, which are responsible for maintaining a lot of those bodily functions that run in the background—like digestion. And to help them do that, they’re made out of what’s known as smooth muscle. When any type of muscle contracts, two main bundles of proteins—actin and myosin—slide past each other and shorten the muscle.

But with regular skeletal muscle, like in your biceps and quads, that process takes more energy. It all starts when calcium ions flow into the muscle cell and reshape the protein troponin, exposing a spot on the actin strand that myosin really wants to grab onto. With a little bit of added energy, the myosin strand cocks back, grabs onto that actin’s sweet spot, and swings forward, shortening the muscle.

Repeat this process for a few other muscle fibers and you’ve got yourself a flexed bicep. But in order to keep the muscles contracting, you need energy in the form of adenosine triphosphate, or ATP. Skeletal muscle uses readily available ATP pretty quickly, so after a bit of exercise your muscles have to pull from other energy sources.

And the byproducts of some of those energy sources—whether it’s lactic acid, hydrogen ions, or something else—are thought to contribute to that feeling of tiredness. But smooth muscle has a few built in safeguards against fatigue. It has much more actin than myosin when compared to a skeletal muscle, and it has no troponin, so the myosin strand always has a spot to hook onto on the actin strand.

That means when smooth muscle contracts, it can stay contracted for a long time without using much ATP, so it won’t run out of fuel. Since the muscle is so energy efficient, it doesn’t need to rely on those alternative sources of energy, which means it won’t produce those metabolic byproducts or get fatigued. This ultra ATP efficiency might sound like a fantastic evolutionary advantage, but there’s a trade-off: it comes with a slower contraction speed, which is not something you’d want in, say, your leg muscles.

You’d have a much harder time running away from a potential predator. Because sometimes you need your leg muscles to kick a shark in the face. But when it comes to the inside of your butt, or your other sphincters, the slower contraction speed is worth being able to clench for a good long while.

Now, most smooth muscles act involuntarily, but hopefully, you have say in when you go to the bathroom. That’s because you actually have multiple layers of anal sphincters: both internal and external. The internal layer contracts involuntarily, but it relaxes when it gets the “gotta go” signal from your body.

The external sphincter is technically skeletal muscle, so it can contract voluntarily, which you’re familiar with if you’ve ever heard nature’s call a little too loudly. This also means that it can get tired, but luckily, it only really plays a backup role to your internal sphincters. And to that end, we owe our sphincters a big thank you.

It's also just a fun word to say. Thanks for watching this episode of SciShow, and thanks to our patrons on Patreon who keep these answers coming. We would not be able to make this show without you!

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