The session begins with a mobility-focused warm-up. In under five minutes, Mack leads you through 90-90 hip switches, world’s greatest stretch, downward dog with hand-release pushups, and deep squats.

THERE ARE TWO kinds of garage gyms. One is a nominal gym in your home, the kind that has all the equipment you need to get a workout in—but also has shaky lighting, mismatched dumbbells, and the floor ...