When I was a kid, there were a number of popular books touting the benefits of isometric exercise. It seemed like a great idea, exercise without equipment, sweat, or repetition. As I remember them, the books would feature highly muscled individuals performing the exercises. Now, I realize that these people must have gained their muscular physiques from workouts with weights, but at the time I was quite convinced.
Naturally, I have become suspicious of claims that you can exercise without exertion. But this article by Dr. Mirkin suggests that passive exercise may have benefits for otherwise inactive people:
The exciting new concept is that passive exercise — sitting on a motor-driven stationary bicycle and letting the pedals move the person’s legs for 30 minutes — burns extra calories and lowers blood sugar and insulin levels in inactive people (Med Sci Sprts Ex, Sept, 2016;48(9):1821-1828). Having their legs moved by motor-driven pedals increases insulin sensitivity by lowering blood sugar rises after eating.