I go to a spinning class on a Keiser M3i bike. My HR is normally 130-170 when spinning, but when I track the activity on my Garmin Vivomove HR, the Heart Rate displayed rarely reaches 110. I have tried refitting the watch, cleaning the sensor, but it is rarely reliable.
Today I solved the issue, or at least identified it. I use the Keiser M3 app on my phone to communicate with the bike via Bluetooth LE during the session. The same phone also runs the Garmin Connect app.
My HR was reading about 100 on the Vivomove, even though I had warmed up and was working fairly hard, when I tried turning off the Bluetooth on my phone. Immediately, the displayed HR on the watch leapt to 139. Turning the Bluetooth on and off also showed the same impact again.
I then quit the Keiser app, and turned the Bluetooth back on. This time the HR on the watch was unaffected, and continued to read accurately until the end of the session. Here is the graph:
I don't know whether the issue is my phone, or the Connect app, but it appears that the watch is not able to monitor the HR and communicate with the phone while the Keiser app is talking to the bike. At least I know that my watch is not faulty.