That's

There's even a muscle that's purpose is to tighten things up when we're sitting or standing to prevent accidents.

Is this a new genitive form, the genitive of that? Wouldn't whose have sufficed? But there's another strange thing in this article (I didn't read all of it): the where (you're forming an angle between the where the poop is and where the poop's gotta come out). So I don't know anymore what to think. I thought at first the authors could be non-native writers of English, but their names do not suggest that. Maybe just a poorly revised text?