At 24, Brian Jaffe got a job in the entertainment industry, sitting behind a desk all day. In just a few short years, his physique was not what it used to be. “I got out of shape,” says Jaffe, now 28.

Apple’s lab rats wear Lululemon. For the past two years, Apple paid people to exercise in a top-secret testing center wearing elaborate monitoring gear that made them look like Bane at a CrossFit gym.

For more than a decade CrossFit has been the dominant sport of fitness, with its competitions combining weightlifting, ...