GHD sit-ups are the type of advanced abs exercise reserved for those in the gym who like to send their core muscles into meltdown. Think of them as the sister of the sit-up that likes to show off.

It’s kind of strange, if you think about it, that the sit-up should be a thing in the first place. In what world would we have to lie down and sit up repeatedly, until our bellies are sore? What are ...