Apple, known for keeping its product developments under the strictest of lock-and-key, gave ABC News exclusive access into its top secret health and fitness lab, where only Apple employees became test subjects for the new Apple Watch.

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.

Jeff Williams, Apple's senior vice president of operations noted that the employees "knew they were testing something, but they didn't know it was for the Apple Watch." We hooked them up with all the masks and so forth, but we would put on an Apple Watch covered up."

Watch the full story on "Nightline" tonight at 12:35 a.m. ET

The lab, according to Apple's Jay Blahnik, also used "climate chambers," to have fitness participants test the watch in different environments, and then they would actually have employees go to different places around the world.

He added that "We have traveled to Alaska and gone to Dubai to really test Apple Watch in all those environments, but we also wanted to be able to have a controlled environment here where we could see those extremes."

Dr. Michael McConnell, a professor in cardiovascular medicine at Stanford Medicine who also directs Stanford's cardiovascular health innovation program, said the Apple Watch will be a game changer in cardiovascular technology.

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