ABC News:

Apple engineers, managers and developers have been secretly volunteering for the past year in this state-of-the-art lab to participate in rowing, running, yoga and many more fitness activities in order to collect data for the Apple Watch’s inner workings.

“[The employees] knew they were testing something, but they didn’t know it was for the Apple Watch,” said Jeff Williams, Apple’s senior vice president of operations. “We hooked them up with all the masks and so forth, but we would put on an Apple Watch covered up.”

You can watch the video of this story tonight on “Nightline” at 12:35 a.m. ET.