When it comes to exercise, there are plenty of iOS apps that can do everything from tracking your running or biking speed to estimating calories burned, but when it comes to ensuring your workouts are worth it, no metric is more important than your hear rate. With that in mind, fitness tech company 4iiii Innovations is introducing the Viiiiva heart rate sensor, complete with iPhone compatibility. Engadget reports that the new device will hit 4iiii's online store and other retailers soon, with a pr
