By Tobias Carroll What do you think of when you imagine an extreme sport? It’s a phrase that’s often used but rarely quantified. Is an extreme sport extreme because of the element of risk involved? Is it more about the ethos with which participants approach things? Or is there some other ineffable quality that eludes easy categorization? And — relevant to the purposes of this article — is it possible for sitting to become an extreme sport? If you were to ask Robert Silk that question, he’d likely reply in the affirmative. A new article in Atlas Obscura chronicles his efforts to take sitting wh…