Lately, my elder millennial body's been feeling extra . . . elderly. My back's achy, my knees make these weird creaky sounds. I could blame my age, but it's not just that: I had a second baby last ...

What is the first thing that pops into your mind when you hear the word “exercise”? Hours in the gym—squatting, curling, lunging, and sweating. What if exercise does not have to involve any of these?

An exercise that takes a more mild approach while offering all the fat-burning benefits of a more grueling workout might sound too good to be true. But some claim that the trending “Zone 2” training, ...