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?

If you’ve ever held a plank, paused at the bottom of a squat or pressed your palms together in front of your chest, you’ve done an isometric exercise. Ta-da! These holds might look simple — after all, ...