For a nation of supposedly obese, lazy, and sedentary layabouts, American consumers sure are interested in tracking their daily activity levels. In 2015, they bought 13.4 million dedicated activity trackers, up 50% from the previous year, and spent almost $1.5 billion on the devices. That’s in addition to the hundreds of millions of smartphones in circulation that also track your daily steps, sleep quality and duration, and calorie expenditure. From FitBit to Jawbone to Apple Watch to dozens of others, the wearable fitness-tracking gadget industry is growing quickly. Venture capital has responded, pouring billions into the wearable industry.