In the first quarter of 2016, Americans bought 19.7 million fitness wearables, an increase of 67 percent over the previous year. By 2020, the global market for fitness-focused apps and devices is expected to grow to $30 billion. This means that more than ever, we're looking at our wrists not only to check the time but also to see how much we've moved, monitor our heart rate, and see how we're stacking up against yesterday's tallies. As a result of...