Whether you’re just counting steps or training for a triathlon, here’s what you need to amp up your fitness routine. $2.1 billion. That’s how much Google will pay for Fitbit, which has only been around since 2007, but is a star in the fitness-tech world. And it’s hardly the only one: Garmin, a company founded in 1989 and once known for GPS devices, saw 28 percent growth in its fitness division from the third quarter of 2018 to the same period in 2019. And just a few years ago in 2015, fitness clothing maker Under Armour bought MyFitnessPal, a free app that does little more than let users count calories. The price: $475 million. There’s big money behind fitness tech—and for good reason. The market for fitness technology is vast, strong, and growing. It encompasses everything from weight-loss apps to socks with built-in running sensors. Fitness tech has gone mainstream, as wrist devices incorporate heart rate monitors, step counters, and sleep-detecting technology. Your bathroom scale can now check your heart health, and your full-length mirror can use augmented reality to become a virtual exercise class. I’ve reviewed lots of products in all t...