Heart icon on watch fluttering

Hi

For the best performance from the heart rate sensor (as well as the Taptic Engine and Wrist Detection), check that you are wearing your watch with a good fit. Wearing it with the right fit (neither too tight nor too loose, with room for your skin to breathe) should be sufficiently comfortable, whilst also allowing the sensors to operate properly:

More information:

Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support

Were you at rest when you looked at the heart rate glance and the icon fluttered? What I'm seeing on my Apple Watch is that repeated, periodic movements severely affect the heart rate reading, even when the fit of the band is just right and the correct heart rate is shown while at rest. For example, when I'm on an indoor rower, my heart rate always shows as between 70-80 bpm which is totally unrealistic - it should be closer to 150 bpm or so. When performing other cardio exercises such as running or cycling, the correct heart rate is shown.