Interestingly the Forerunner 225, the first watch from Garmin with an embedded heart rate monitoring system was featuring a heart rate technology developed by Mio Global (based in Vancouver, Canada); “an award-winning performance wearables brand acclaimed for EKG accuracy demanded by athletes everywhere,“ Garmin wrote at the time. The Forerunner 225 was launched in May.
In September TomTom also had launched a GPS watch with an embedded heart rate, featuring their own proprietary technology. They previously also used technology from Mio Global in their Runner Cardi GPS sport watch launched in April 2014.
The wrist-based heart rate systems use an optical sensor which monitors changes in blood flow by shining light through the skin.
Several reviews proved that chest straps are more accurate than wrist-based heart rate monitor technology. (read here: http://www.wareable.com/fitness-trackers/heart-rate-monitor-accurate-comparison-wrist)
However its is sure that it is far less cumbersome hence why the TomTom Cardio was a success and why Garmin is turning to wrist-based technology since May (although the company also unveiled today the Forerunner 230 that as a chest strap as opposed to the Forerunner 235).
However it is likely that this technology will improve. As a matter of fact the new watch from Garmin seems to feature three optical sensors instead of the two found in most Mio products and Garmin previous watch.