From reading the Vivosmart HR (VHR) subforum on Garmin forums, I learned from a supermoderator that:
1) Intensity minutes can be triggered by arm movement/heart rate increases
2) There is no magical formula for how fast, but it requires significant heart rate increase (70% of maximum was one theory offered) to get the meter running.
3) People have earned intensity minutes on stationary bikes.
I tried 15 minutes on a bike earlier this week, getting my HR averaging 118 to a max of 139, but failed to earn any minutes.
20 minutes on the arc elliptical earned me minutes, with an average HR of 108 and a maximum of 139 the same day.
So, either the bike session was too short, non-arm movement activity requires higher heart rate to trigger counting of intensity minutes compared to arm swinging like walking or running or elliptical, or...
Today, I did a bike session that was more intense. I got average up to 121, max of 148, and did a bunch of hard one minute intervals thrown in.
And voila - I earned vigorous intensity minutes (vigorous are given 2:1 to moderate minutes according to Garmin Connect) -- I got 55 intensity minutes for 30 minutes of stationary biking - not quite 2:1 but close.
Mr. Science was intrigued - exactly what is the point where it kicks the intensity meter to the point where it starts counting? And where do vigorous minutes kick in, vs moderate minutes?
So I got on the treadmill and started a slow walk - around 2.5 mph, keeping my hr below 90. Even after 10+ minutes, the IM didn't start counting -- I started and it remained on 391 minutes. But at around 11:21, it started flashing (indicating the meter is either ready to count or actually counting)
A manual and VHR pulse check showed I was bouncing around in the 70s and 80s, so I decided to take things for a spin at minute 29:00 and do 20 at a run pace -- the VHR showed around a 7:55 pace, but it was definitely over 8:00 per mi by feel.
I got my pulse going 125+ for the run but as high as 141 by the end, earning 28 IMs for a 20 minute run -- around a 1.5 ratio, so it though I was doing vigorous minutes for almost half the run which I would say is about right -- I did have to step off to tie my laces at one point.
To answer my questions: I think that you need to plan on 10-15 minutes of walking at a brisk pace, 10+ minutes of running, or around 15 minutes of biking to trigger intensity minutes. You don't have to start an activity for this to happen -- VHR will be activated from arm swing and/or increased H. And for vigorous minutes to kick in, I think the supermoderator's comment that you need to be north of 70% maximum hr is probably right.
And day three of charge #2 comes to an end with still about 1/3rd of the battery life left -- on daily activity creation or hr broadcasting, with notifications, with weather screen, and fairly constant wrist play. Get your minds out of the gutter.
Back to resting (HR), even though Sunday is long run day.
See newer post: Intensity Minutes on a treadmill walk