How is Cardio Load calculated?

[tesb]

I am utterly baffled by the cardio load 'feature' on my FitBit. I get a cardio load score for a light walk where my heart rate hardly goes into the light exercise zone, but nothing at all for a pretty vigorous 25-minute Spin session of constant high heart rate.  Can anyone explain how that can be the case?

As a result i can have had a good cardio workout but still have the app pestering me to increase my cardio load. No sense whatsoever. 

Moderator edit: clarified subject.

[ManuFitbit]

Hi and welcome back to the community @tesb

Thank you for sharing with me those details about the Cardio Load issue. 

Spinning can be tricky for wrist-based sensors. When you’re holding the handlebars, the tension in your wrist can sometimes make it hard for the sensor to 'see' your actual heart rate. If your device doesn't catch those high-intensity peaks, it won't trigger the Cardio Load points, whereas during a walk, your arm's natural movement makes it much easier to get a perfect reading.

Please, the next time move the band a bit higher up your forearm and snug it up just a notch. Also, manually starting the 'Spinning' mode on your wrist tells the sensor to work extra hard to catch every beat.