There are probably hundreds of apps that will turn your iPhone into a mobile fitness device, using the phone’s GPS to track your speed and from there derive calories burned, route taken and so on. Some of them even connect wirelessly through a dongle to heart-rate monitors and the like. It’s like having a coach in your pocket, only he doesn’t wake you up in the morning by shouting at you.
Now, though, things are going to the next level. Wahoo’s new Blue SC speed and cadence sensor talks direct to the iPhone via low-powered Bluetooth 4, letting it communicate directly with your bike.