A:Answer Based on my observation, it seems to be based on sustained elevated heart rate for at least 2 minutes. I haven't had my device require longer than that to start counting Intensity Minutes. And it adequately measured the elevated heart rate each time I have shoveled snow (and measured this period as an Activity), which didn't really involve many measurable steps because my arm movement was focused on the shovel.
How it calculates "intensity" seems to be a combination of factors; it doesn't seem to be a fixed rate for all users. It seems to use what it has measured as your average resting heart rate. But it also seems to factor in your general activity level (in profile settings). My husband easily gets "intensity minutes" just from leisurely walking, but his fitness "level" is lower than mine. We can be walking together at the same pace; he'll get intensity minutes and I will not (even though I'm shorter and working harder to match his stride).
I think there is a page on the Garmin Connect website where you can customize your targets for intensity as percentage of your average resting heart rate, but I haven't done that since this is my first fitness tracker. Sometimes when I take my brisk mid-morning or mid-afternoon walk (each of which is about 10 minutes), it credits 8-10 minutes of the basic intensity rate. This past week, when I was dealing with colder temperatures and deep snow on those walks, it credited those periods with the much higher intensity rate.
