The Beets BLU heart rate monitor transmits the heart rate through Bluetooth to the iPhone, where an app like Runkeeper logs the data. Heart rate monitor with chest strap, in March 2014
Koen Delvaux's blog: Heartrate monitors and GPS for running: the Runkeeper-Magellan Echo dream combination that would beat Polar, Garmin, Tomtom and Suunto
Apple Watch Tracking – ASICS Runkeeper Help Center