Fitbit

This can be seen in the progression of Fitbit's product line. Fitbit's original 2009 product was clipped onto waists, but they quickly moved to wristbands 2 years later–users can easily interact with their wrists for inspiration, but not with their waists. While early models simply had displays, more recent Fitbit devices now automatically vibrate periodically throughout the day to remind users to exercise, and vibrate celebratorily when daily goals are met. Fitbit also now sends weekly summary emails to users, trying to inspire through a medium outside the tracker itself.
Without this design transition from passive to active, activity trackers may never have become as mainstream as they are today.