In the fitness studio behind his home, Tony Horton grabbed a chin-up bar and lifted himself, attempting to execute a routine called the gyrating lever. The first part—the lever—came easily enough, his legs rising until his body formed a tight line parallel to the ground.

But when he tried gyrating—twisting side to side—nothing happened. He dropped to his feet and grimaced. "I didn't get it," he said, rubbing his hands. "Not even close."

As...