The Wahoo key and app are compatible with ANT+ sensors such as cycling power meters and cadence sensors
With the Wahoo Key, a compatible app and ANT+ sensors your iPhone can act as the hub of your exercise tracking. Replacing your dedicated HRM watch, cycle computer, GPS watch, etc.
I found the Wahoo key by chance when I saw it mentioned in the update for Walkmeter. I researched at wahoofitness.com, purchased it and the Garmin foot pod from Amazon.
I use this setup everyday. I highly recommend you research this setup for your own exercise routines. My favorite use case is tracking Treadmill workouts.