The Activity app is passive, meaning it always records data. Third party apps only write to the Health App and do not have access to the heart beat monitor.
The Workout app will constantly record heart rate when the app is in use. Right now i'm using three different apps for running at the same time. Nike+ and Zombies Run on an iPhone 6+ and on the watch I am using Workout. I've only gone on two runs and both times I used Nike+ and Workout (Zombies Run I did for the second run as another measure)
There was some differences in the measurements due to me starting the apps at different times and the Activity app has a countdown timer of 3 seconds. I didnt want to try using Nike+ on the watch because I don't think the watch can multitask with Workout and Nike+ at the same time.
Hopefully WWDC unleashes the SDK for native watch apps and allows heart rate monitoring for 3rd party. Apple's app does not write data to 3rd party apps and 3rd party apps only write data to Health. However I do believe the phone is smart enough to just take the data from all active apps instead of duplicating data. (I read on another thread where someone was getting duplicate data, might have been their settings) The biggest difference on my initial observation is the calories burned on Nike vs Workout....not sure if its because Nike counts elevation or I need to calibrate or just the time difference in starting the apps (pretty much all 3 apps were started one after the other)
