Earlier this month, when Google updated one of its premiere Android applications, it quietly added a personal activity tracker.

Google Now is an Android-only app that uses Google services to smartly deliver users various kinds of information to help them throughout their day - without them having to ask. When Google updated the application at the beginning of December, they didn't mention the feature, called the "Activity Summary" in their official blog post, but the sharp eyes at MIT Technology Review picked up on it.

There's only minimal information about the feature, one of 25 "cards" within the app, available on the Google Now page. It delivers a monthly activity report of time spent walking or cycling, measured passively by the phone itself, and displays it alongside the previous month's data for comparison.

Google's history with personal health monitoring is somewhat dramatic. The company launched Google Health, a personal health record, in 2008. The service was discontinued in 2011 because the product failed to scale beyond a relatively small group of dedicated patient users, and also because new CEO Larry Page wanted to re-focus the company. The failure of Google Health has become a major case study for developing digital consumer health services.

Although Google Health wasn't designed to be an activity tracker, it did have an open API that allowed it to integrate with trackers like Fitbit and the Withings weight scale. The activity tracker built into Google Now is a subtle addition, included with almost no fanfare. But it could be a way for Google to test the waters before making another entry into the world of mobile health.

GigaOM suggested that Google Now could be paving the way for Project Glass, a Google initiative to put the functionality of a smartphone into a pair of glasses with a heads up display. Project Glass has been a big driver of interest in the wearables market, which still mostly consists of activity trackers. If Google Glass functions as a tracker at launch, that could be cause for worry for makers of wearable trackers.